Onion enzyme system

An active glutathione S-transferase system was detected in the onion enzyme system when it was assayed with [ C]PCNB and GSH (9.). An initial rate of 14 nmol product/mg protein/hr was observed and a yield of 18 was obtained in 17 hr. HPLC indicated that S-(PCP)GSH was the only major conjugated product of this reaction. This was consistent with the Iji vivo studies with onion that showed that S-(PCP)GSH was the dominant GSH conjugate formed. In contrast, an enzyme from pea produced S-(PCP)GSH, S-(TCNP)GSH, and what appeared to be two isomeric S,S -(TCP)diGSH conjugates (6 ). 

The immediate precursor of pentachlorothioanisole was assumed to be pentachlorothiophenol. The onion enzyme system was assayed for pejjtachlorothioanisole... 

Methyltransferase enzymes are widespread in the plant kingdom, but they are frequently very substrate specific. The substrate specificity of the methyl transferase from the onion enzyme system was tested with 18 different substrates (Table I). Pentachlorothiophenol was the best substrate tested however, four other substrates showed high levels of activity and only nine substrates showed less than 5% of the activity of pentachlorothiophenol. The three most active substrates were ortho substituted thlophenols. 

An vitro enzyme system from onion was used to show that this pathway was operative in plants ( , ). Onion was chosen as the source of enzymes because pentachlorothioanlsole was an important metabolite of PCNB in onion ( ). 

Members of the genus Allium (onion, garlic, leeks, etc.) provide a powerful example of an enzyme which distinguishes itself. Though there are hundreds of detectable enzymes occurring naturally in foods, it is difficult to find many examples where an individual enzyme is predominately responsible for a phenomenon. Enzymes and enzyme systems are responsible for the very identity of the food product. It is the enzyme composition that determines the flavor attributes and the textural differences in firmness, as well as the general appearance and color of food items. Table II provides examples of cases where particular changes in food can be primarily ascribed to a specific enzyme or enzyme system. 

The deamination of xenobiotic cysteine conjugates can lead to thlo-acetlc acid conjugates, as observed with PCNB In peanut (100). or thlo-lactlc acid conjugates, as observed with EPTC In corn and cotton (106) or PCNB In peanut (WO). Cysteine conjugates can also undergo 8-lyase cleavage to thioalcohols, as observed with PCNB In peanut (WO). The thioalcohols can be methylated by an S-adenosyl-methlonlne methyltransferase system, as demonstrated with an In vitro enzyme system derived from onion root (J01 ). Pentachloro-... 

Examples of phytochemicals and their biological effects include isothiocyanates and sulforaphane which are found in vegetables such as broccoli and have been shown to trigger enzyme systems that block or suppress cellular DNA damage and reduce tumor size in animal studies (4) Allylic sulfides, found in onions and garlic, can enhance immune function, increase the production of enzymes that help to excrete carcinogens, decrease the proliferation of tumor cells, and reduce serum cholesterol levels (5). Isoflavonoids in soy have also been shown to reduce serum cholesterol levels in humans (6). 

Sulfinylpropene is the main tear-inducing compound (technical term lachrymator) released when you slice onions. The most interesting part of this story is that this chemical is not present in onions before you slice them. In fact, the generation of this tear-inducing molecule is not a mistake, but part of the defense system that plants have developed to deter animals from continuing to eat them. When you cut, or an animal takes a bite of, an onion, an enzyme, alliinase, that is normally safely stored within the plant s cells is released and starts to wreak havoc. Alliinase converts sulfoxides to sulfinyl groups, which makes you cry like a baby. 

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