On the basis of the relationship between HOEtVal levels and exposure levels of ethylene or ethylene oxide, an &#;uptake&#; (i.e. amount metabolized) of 1 mg ethylene/kg bw was calculated to be equivalent to a tissue dose of ethylene oxide of 0.7 × 10 &#;6 mol × h/L [0.03 mg × h/kg bw] ( Törnqvist et al. , ). This value is in agreement with the value of 0.5 × 10 &#;6 mol × h/L that can be calculated from the pharmacokinetic data for ethylene and ethylene oxide published by Filser et al. () .

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Nonsmoking fruit store workers exposed occupationally to atmospheric ethylene (0.02&#;335 ppm [0.023&#;3.85 mg/m 3 ]) used for the ripening of bananas had levels of 22&#;65 pmol/g Hb HOEtVal, whereas nonsmoking controls had 12&#;27 pmol/g Hb. On the basis of a mean exposure concentration of 0.3 ppm [0.345 mg/m 3 ], it was estimated that about 3% (range, 1&#;10%) of inhaled ethylene was metabolized to ethylene oxide ( Törnqvist et al. , a ). This percentage is equal to the alveolar retention at steady state calculated from inhalation pharmacokinetics (see above). The two values are in agreement. An increment of 100&#;120 pmol/g Hb HOEtVal was estimated for a time-weighted average exposure (40 h/week) to 1 ppm ethylene [1.15 mg/m 3 ] ( Kautiainen & Törnqvist, ; Ehrenberg & Törnqvist, ).

Exposure to environmental ethylene concentrations of 10&#;20 ppb [11.5&#;23 µg/m 3 ] was associated with an HOEtVal increment of 4&#;8 pmol/g globin at steady state ( Törnqvist & Ehrenberg, ). Background levels of HOEtVal were predicted on the basis of pharmacokinetic parameters of ethylene and ethylene oxide, together with the rate constant of the reaction of ethylene oxide with the N-terminal valine in Hb and confirmed by measured data. HOEtVal levels resulting from endogenous ethylene only were calculated to be about 12 pmol/g Hb. Those resulting from both endogenous and environmental ethylene (15 ppb [17.25 µg/m 3 ]) in the area of Munich (Germany) were computed to be about 18 pmol/g Hb; the measured level was about 20 pmol/g Hb and, hence, in close agreement with that predicted ( Filser et al. , ). No difference in HOEtVal adduct levels was seen in nonsmoking workers in an ethylene plant and in nonsmoking controls not occupationally exposed ( van Sittert & van Vliet, ).

Endogenous production of ethylene can be deduced from its exhalation by unexposed subjects ( Ram Chandra & Spencer, ; Shen et al. , ; Filser et al. , ). For a man of 70 kg, a mean production rate of 32 nmol/h [0.9 µg/h] and a corresponding mean body burden of 0.011 nl/ml tissue [equivalent to 0.44 nmol/kg bw or 0.012 fig/kg bw] was calculated for ethylene gas ( Filser et al. , ). The amount of ethylene in the breath of women is increased significantly at the time of ovulation; no difference was observed in the basal ethylene outputs of non-pregnant and pregnant women and of men ( Harrison, ). The biochemical sources of ethylene are unknown; however, several mechanisms by which it might be produced in mammals are discussed below.

Uptake of ethylene into the body is low. Clearance due to uptake, which reflects the transfer rate of ethylene from the atmosphere into the body, was 25 L/h for a man of 70 kg. This value represents only 5.6% of the experimentally obtained alveolar ventilation rate of 450 L/h. The majority (94.4%) of ethylene inhaled into the lungs is exhaled again without becoming systemically available via the blood stream. Maximal accumulation of ethylene in the same man, determined as the thermodynamic partition coefficient whole body:air (K eq = Conc animal /Conc air ), was 0.53. The concentration ratio at steady state was even smaller (0.33), owing to metabolic elimination. Clearance due to metabolism, in relation to the concentration in the atmosphere, was calculated to be 9.3 L/h for a man of 70 kg. This indicates that at steady state about 36% of systemically available ethylene is eliminated metabolically and 64% is eliminated by exhalation as the unchanged substance, as can be calculated from the values of clearance of uptake and of clearance of metabolism. The biological half-life of ethylene was 0.65 h. The alveolar retention of ethylene at steady state was calculated to be 2% ( Filser et al. , ). From theoretical considerations of the lung uptake of gases and vapours ( Johanson & Filser, ), it can be deduced that the low uptake rate of ethylene is due to its low solubility in blood: Ostwald's solubility coefficient for human blood at 37 °C, 0.15 ( Steward et al. , ).

The inhalation pharmacokinetics of ethylene have been investigated in human volunteers at atmospheric concentrations of up to 50 ppm [57.5 mg/m 3 ] by gas uptake in a closed spirometer system ( Shen et al. , ; Filser et al. , ). The uptake, exhalation and metabolism of ethylene can be described by first-order kinetics.

4.1.2. Experimental systems

Four male CBA mice (average body weight, 31 g) were exposed together for 1 h in a closed glass chamber (5.6 L) to 14C-ethylene (22 mCi/mmol) in air at 17 ppm×h [22.3 (mg/m3)×h, equivalent to about 1 mg/kg bw]. Blood and organs from two mice were pooled 4 h after the end of exposure. Radioactivity was about the same in kidney (0.16 µCi/g wet weight) and liver (0.14 µCi/g) but lower in testis (0.035 µCi/g), brain (0.02 µCi/g) and Hb (0. µCi/g Hb). Urine was collected from the two other mice during 48 h, and blood was collected after 21 days. S-(2-Hydroxyethyl)cysteine was identified as a metabolite of ethylene in urine (3% of 14C in urine) by thin-layer chromatography. The radioactivity in Hb was 0.011 µCi/g Hb. These data, together with those on specific hydroxyethyl derivatives at amino acid residues of Hb (see below), indicated that ethylene was metabolized to ethylene oxide (Ehrenberg et al., ).

In several experiments, disposition of 14C-ethyIene (free of 14C-acetylene or &#; 97% pure) in male Fischer 344 rats (170&#;220 g) was determined over 36 h following 5-h exposures in a closed chamber (35 L) to 10 000 ppm [ mg/m3]. In each experiment, up to four rats were exposed together in a single chamber. Within about 1 min after the end of exposure, animals were transferred to individual all-glass metabolism cages. Most of the eliminated 14C was exhaled as ethylene (18 µmol [504 µg] per rat exposed to acetylene-containing ethylene); smaller amounts were excreted in urine (2.7 µmol ethylene equivalents/rat) and faeces (0.4 µmol) and exhaled as CO2 (0.16 µmol). Radioactivity was also found in blood (0.022 µmol ethylene equivalents/ml), liver (0.047 µmol ethylene equivalents/liver), gut (0.034 µmol ethylene equivalents/gut) and kidney (0.006 µmol ethylene equivalents/kidney). Pre-treatment of animals with a mixture of polychlorinated biphenyls (Aroclor ; 500 mg/kg bw; single intraperitoneal injection five days before exposure) had no measurable influence on ethylene exhalation but resulted in a significant (p < 0.05) increase in exhaled 14CO2 (2.04 µmol ethylene equivalents/rat) and of 14C in urine (11.1 µmol ethylene equivalents/rat) and in blood (0.044 µmol ethylene equivalents/ml). The organ burden of 14C was one to two orders of magnitude greater in Aroclor -treated than in untreated animals. Radioactivity also became detectable in lungs, brain, fat, spleen, heart and skeletal muscle. The data were interpreted as indicating that the metabolism of ethylene can be stimulated by an inducer of the mixed-function oxidase system (Guest et al., ).

The pharmacokinetics of inhaled ethylene have been investigated in male Sprague-Dawley rats using closed exposure chambers in which the atmospheric concentration-time course was measured after injection of a single dose into the chamber atmosphere (Bolt et al., ; Bolt & Filser, ; Shen et al., ; Filser, ). Uptake of ethylene into the body was low. Clearance due to uptake (as described above) was 20 ml/min for one rat of 250 g, which represents only 17% of the alveolar ventilation (117 ml/min; Arms & Travis, ). Most (83%) inhaled ethylene that reaches the lungs is exhaled again without becoming systemically available via the blood stream. Maximal accumulation of ethylene in the organism, determined as the thermodynamic partition coefficient, whole body:air (Keq = Concanimal/Concair), was 0.7. The concentration ratio at steady-state whole body:air was somewhat lower owing to metabolic elimination, and it decreased from 0.7 to 0.54 at exposure concentrations below 80 ppm [92 mg/m3]; however, at very low atmospheric concentrations, the concentration ratio at steady-state whole body:air increased again, owing to endogenous production of ethylene: For instance, it was almost twice the value of the thermodynamic partition coefficient whole body:air at an exposure concentration of 0.05 ppm [0.06 mg/m3] (calculated using the pharmacokinetic parameters and equation 18 of Filser, ). At concentrations between 80 and 0.1 ppm [92 and 0.12 mg/m3], clearance was seen, due to metabolism related to the concentration in the atmosphere of about 4.7 ml/min for a 250-g rat. In that concentration range at steady state, therefore, about 24% of systemically available ethylene is eliminated by metabolism and 76% by exhalation of the unchanged substance (taking into account values of clearance of uptake and clearance of metabolism). The alveolar retention of ethylene at steady state was 3.5%, and the biological half-life was 4.7 min (Filser et al., ). At atmospheric concentrations greater than 80 ppm [92 mg/m3], metabolism of ethylene became increasingly saturated, reaching a maximal rate of metabolism (Vmax) of 0.035 µmol/(min×250 g bw) [0.24 mg/(h×kg bw)] at about ppm [ mg/m3]. The apparent Michaelis constant (Km) related to the average concentration of ethylene gas within the organism was 130 nl/ml tissue, which corresponds to an atmospheric concentration of 208 ppm [239 mg/m3] at Vmax/2, calculated by means of the kinetic parameters given by Filser ().

Gas uptake studies with male Fischer 344 rats gave values for Vmax of 0.24 mg/(h×kg bw) and an &#;inhalational Km&#; (related to the atmospheric concentration) of 218 ppm [251 mg/m3] (Andersen et al., ).

Involvement of cytochrome P450-dependent monooxygenases in the metabolism of ethylene in male Sprague-Dawley rats was suggested by the complete inhibition of metabolic elimination after intraperitoneal treatment with 200 mg/kg diethyldithiocarbamate 15 min before exposure and by an increase in the rate of its metabolism with a Vmax of about 14 µmol/(h×kg bw) [0.33 mg/(h×kg bw)], after treatment with a single dose of Aroclor (500 mg/kg bw) six days before the experiment (Bolt et al., ).

The metabolism of 14C-ethylene in 15 male CBA mice kept for 7 h in a closed exposure chamber (11 L), in which the atmospheric concentration-time course was measured after generation of an initial atmospheric concentration of 10 ppb [11.5 µg/m3], was reduced by co-exposure to propylene at ppm [ mg/m3], suggesting inhibition of ethylene metabolism by propylene (Svensson & Osterman-Golkar, ).

In liver microsomes prepared from male Sprague-Dawley rats, ethylene at concentrations of up to 10% [115 g/m3] in the gas phase was metabolized to ethylene oxide in the presence of an NADPH regenerating system (1 h, pH 7.5, 37 °C). The rate of formation of ethylene oxide was saturable (Vmax, 0.67 nmol/h per mg protein) and could be reduced by the addition of diethyldithiocarbamate or β -naphthoflavone to the microsomal suspension. Treatment of the rats with phenobarbital (single intraperitoneal injection of 80 mg/kg bw followed by three days of 0.1% in drinking-water) before preparation of liver microsomes did not change the Vmax (Schmiedel et al., ).

Male Sprague-Dawley rats exposed to ethylene exhaled ethylene oxide. In these experiments, two animals were kept together up to 21 h in a closed exposure chamber (6.4 L). The concentration of ethylene in the atmosphere of the chamber was maintained at greater than ppm [ mg/mg3] by repeated additions, in order to maintain Vmax conditions for ethylene. One hour after the beginning of exposure, the atmospheric concentration of exhaled ethylene oxide reached a peak value of 0.6 ppm [0.69 mg/m3]. After about 2.5 h, the concentration had decreased to about 0.3 ppm [0.345 mg/m3] and then remained constant. On the basis of the concentration-time courses of atmospheric ethylene, it was speculated that this decrease was due to rapid induction of ethylene oxide metabolizing enzymes, whereas the rate of ethylene metabolism remained unaffected (Filser & Bolt, ). In male Sprague-Dawley rats exposed to concentrations greater than ppm, the amount of ethylene taken up per unit time from the atmosphere of a closed chamber remained constant over exposure times of up to 30 h (Bolt et al., ). Pharmacokinetic data for ethylene and ethylene oxide indicated that under steady-state conditions only 29% of metabolized ethylene is available systemically as ethylene oxide. Therefore, assuming that the liver is the principal organ in which ethylene is metabolized, an intrahepatic first-pass effect for the intermediate ethylene oxide was suggested (Filser & Bolt, ).

In view of the saturability of ethylene metabolism, the maximal possible average body concentration of its metabolite, ethylene oxide, was calculated to be 0.34 nmol/ml tissue [15 µg/kg bw] in an open exposure system (infinitely large atmospheric volume). The same value was computed to result from exposure to ethylene oxide at an atmospheric concentration of 5.6 ppm [10.2 mg/m3] at steady state (Bolt & Filser, ).

Ethylene oxide was found in the blood of male Fischer 344/N rats during exposure to an atmospheric ethylene concentration of 600 ppm [690 mg/m3]. A maximal value of about 3 µg/g blood of ethylene oxide was seen 8 min after the start of exposure to ethylene; this value was followed 4 min later by an immediate decrease to about 0.6 µg/g, and the level remained constant for the following 46 min. During exposure, the cytochrome P450 content in the liver was reduced to 94% after 20 min and to 68% after 360 min. It was speculated that an ethylene-specific cytochrome P450 isozyme was rapidly deactivated during exposure to ethylene, resulting in reduced formation of ethylene oxide (Maples & Dahl, ). This speculation is based on results obtained by an unspecific method for the determination of cytochrome P450 which is not suitable for the determination of cytochrome P450 isozymes; however, under certain conditions, suicide metabolism of ethylene in rat liver does seem to occur, as indicated from experiments of induction of cytochrome P450-dependent monooxygenases. In male Sprague-Dawley rats treated with phenobarbital (80 mg/kg bw, intraperitoneal injection daily for four days, exposure to ethylene on day 5) and then exposed for 3 h to a mixture of commercial ethylene (contaminated with about 10 ppm acetylene) and air (1:1 v/v), a green pigment was found in the liver 4 h after exposure. The same pigment was formed in vitro during incubation of acetylene-free ethylene with the × g supernatant of a rat liver homogenate (from phenobarbital-pretreated animals) in the presence of NADPH. No controls were used (Ortiz de Montellano & Mico, ). The pigment was identified as a N-(2-hydroxyethyl)protoporphyrin IX, an alkylation product of the prosthetic haem of cytochrome P450-dependent monooxygenases. It was concluded that the phenobarbital-inducible form of cytochrome P450 was destroyed during oxidative metabolism of ethylene (Ortiz de Montellano et al., , ).

The further metabolic fate of ethylene oxide is described in the monograph on that chemical.

Ethyelene &#; A Useful Hydrocarbon for Agriculture

Ethylene (C2H4) is a small hydrocarbon, which is colorless and odorless. This gaseous phytohormone that is produced by plants has various valuable applications in the agriculture industry. It can be both beneficial and harmful because it promotes and inhibits plant growth and development at various stages in a plant&#;s life. It is, however, best known for its ripening effect on fruit.

How Ethylene Acts

Ethylene has three pathways which produce a variety of effects on plants depending on their sensitivity to it and their life stage. The three modes of actions are:

• Ripening: Ethylene is released in high concentrations by fruit that is ripening. It acts by:
  • Dissolving pectin and softening the fruit,
  • Changing the color of the fruit, and
  • Converting stored starches and acids into sugars, making the fruit sweet.

Once ripening is underway, it, in turn, triggers the production of more ethylene to continue the process of ripening.

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• Senescence: Ethylene brings about ageing, or senescence, by triggering programmed cell or organ death. This is not as terrible as it seems, as the organism discards old cells or unnecessary parts to make way for new cells or the next stage of growth. An example of this is abscission. Ethylene is also produced for the purpose of starting senescence when any part of the plant is damaged.
• Response to stress: Ethylene production is increased as a response to stress. When this happens, it can cause inhibition of stem elongation, stem thickening, and epinasty, the bending or curving of stems or roots to avoid obstacles.

Effects of Ethylene

Ethylene can affect all parts of a plant as a result of its three pathways.

• Germination: It breaks seed dormancy and promotes germination, for example in groundnuts.
• Sprouting: Ethylene can trigger the sprouting of buds in tubers when its levels increase, as seen in potatoes.
• Root development: It encourages the growth of adventitious roots on the stems of some vegetables such as tomatoes.
• Stem: It promotes elongation of petioles and internodes, which helps paddy seedlings to emerge quickly out of the water.
• Leaves: When leaves grow old, ethylene removes chlorophyll from leaves, causing them to change color. This is followed by abscission, or the falling of leaves.
• Flowers: Ethylene causes wilting in flowers and their abscission. Conversely, it also promotes flowering in fruits and vegetables.
• Fruit: It is best known commercially for its action on fruit, both in ripening them and in causing abscission.

Application in Food Production

Ethylene levels are an important consideration in agriculture due to ethylene&#;s positive applications and the problems it creates for the food industry.

Beneficial Effects

Ethylene can increase profits by extending the time from harvest to the shop, and by improving the quality and quantity of food. The gas can be delivered through ethephon (a liquid), a cylinder, or a catalytic generator.

• Ripening fruit: Thanks to ethylene, mature fruit is harvested before ripening. When it is ready to be sold, fruit is ripened artificially with ethylene. Besides ripening, ethylene brings about color changes in citrus fruit through the removal of chlorophyll.
• Increasing flowering: Ethylene is used to promote the formation of female flowers in cucumbers to increase overall yield.

Problems Caused by Ethylene

Being a gas, ethylene spreads out of a piece of fruit and into the environment when it is produced and quickens the ripening of other fruit nearby. Moreover, &#;one rotten apple can spoil the whole basket,&#; if the process continues unchecked.

Not all fruit and vegetables produce or need ethylene. For example, cherries and blueberries do not. However, many temperate and tropical fruits do start to ripen when exposed to heightened ethylene levels.

• Producers: Fruits producing ethylene in their cells are apples, bananas, tomatoes, avocados, melons, pears, squash, and stone fruits such as mangos.
• Sensitive fruit: Plants sensitive to ethylene that do not produce it themselves are broccoli, cabbage, cauliflower, leafy greens, and lettuce.

Measuring Ethylene

In both cases, where ethylene needs to be added or removed to maintain fruit quality, it is necessary to measure levels of ethylene to avoid wasting the gas or to prevent the spoilage of produce, respectively. Different methods can be used to measure ethylene in the air.

1. Micro gas chromatography systems are highly efficient but expensive.
2. Gas detector tubes, which can measure ethylene in the air, comprise a simple handheld system.
3. Electrochemical sensors are available as small but precise handheld instruments. Some examples are:
   1. The F-950 Handheld Ethylene Analyzer, which can detect ethylene levels as low as 0.5 ppm, and can measure between 0 &#; 200 ppm of the gas.
   2. The F- 940, which is used in storage facilities, has a resolution of 0.1ppm, with a range of 0 &#; 10 ppm.
   3. The F-960, used during ripening, has a resolution of 1 ppm, with a range of 0 &#; 500 ppm.

Removing Ethylene

When recorded levels during storage and transport are high, there are many techniques that can be used to control the level of ethylene, such as scrubbing, ventilation, or the use of UV radiation.

1. Scrubbing: Dry chemical scrubbers are the traditional method of removing ethylene. Air is sucked from the bottom of the room where ethylene sinks, as it is heavier than the air, and is then passed through filters with potassium permanganate (KMnO4), which oxidizes and neutralizes ethylene. The cleaned air is released free of the gas. Some systems use ozone instead of potassium permanganate.
2. Absorption Filters: There are simple ethylene absorption filters that can be hung in storage rooms and vehicles to absorb the gas given out by fruits and vegetables.
3. Ventilation: Air in refrigerated containers and rooms is circulated regularly with fans to help the produce and fruits to respire, and remove accumulated ethylene and carbon dioxide. Ventilation increases cooling costs, as the introduced air needs to be cooled to maintain storage temperatures. The rate of ventilation varies depending on the fruit or vegetable.
4. UV radiation: UV radiation is used to remove ethylene, as it reacts with oxygen in the air to produce ozone, which in turn oxidizes the gas.

Control to Avoid Waste

Besides removing ethylene, many measures can be taken to prevent the spread and accumulation of ethylene by separating fruits producing the gas from ones sensitive to it. Other methods include keeping temperatures low, reducing oxygen content, and increasing carbon dioxide levels to inhibit ethylene production in fruits. In every case, precise and regular measurement of the levels of ethylene is essential to monitoring its level and effects.

&#;

Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture

Sources

1. Abbas Zaidiab, N.A., Tahirab, M.W., Vinayakaa, P.P., Luckluma, F., Vellekoopa, M., & Langa, W. (). Detection of Ethylene Using Gas Chromatographic System. Procedia Engineering
168, 380-383. https://doi.org/10./j.proeng..11.140

2. AgraCoTech. (, Oct 13). Ethylene Removal Video. Retrieved from https://www.youtube.com/watch?v=Pn5_OTyYW1Q

3. Bakore, N. [Neela Bakore] (, Aug 5). Plant growth and development. Retrieved from https://www.youtube.com/watch?v=yWLyeSGUejQ

4. Bialigy. (, Mar 3). Plant hormonal control &#; ethylene. Retrieved from https://www.youtube.com/watch?v=_qFCNCnBurw

5. Brecht & Reid. (). Ethylene inhibition and control . Retrieved from
http://ucce.ucdavis.edu/files/datastore/234-.pdf

6. Bry Air Ethylene Scrubber&#; BES Series. Retrieved from https://www.bryair.com/products-solutions/gas-phase-filtration-systems/bry-air-ethylene-scrubber-bes-series/

7. Ethylene (, Dec 16). Retrieved from https://www.youtube.com/watch?v=jpitvOxnM4A

8. Heap, R., & Marshall, R. . Ventilation effects and requirements in containerised refrigerated transport. International Congress of Refrigeration , Washington. D.C. Retrieved from http://www.crtech.co.uk/pages/environmental-testing/fresh-air-ventiir.pdf

9. QA Supplies. Retrieved from https://qasupplies.com/air-gas-analysis/ethylene-testing/

10. Suslow, T.V. Ozone Applications for Post-harvest Disinfection of Edible Horticultural Crops. Extension of Vegetable Crops, University of California, Davis. Retrieved from http://anrcatalog.ucdavis.edu/pdf/.pdf

11. Sylvia Blankenship, S. [Sylvia Blankenship]. (, Mar 15). Ethylene: The ripening hormone. Retrieved from http://postharvest.tfrec.wsu.edu/pages/PCF

12. USDA. (, Feb 28). Ethylene gas. Retrieved from https://www.youtube.com/watch?v=b_UfObml9D4.

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