Ethionine as an inhibitor of enzyme systems mediating choline oxidation.

Ethionine (S-ethylhomocysteine) has been shown to inhibit the growth of rats and of various microorganisms (2-7). In rats, this growth inhibition can be overcome by simultaneous administration of either methionine or choline, indicating that ethionine interferes in some manner with the metabolism of these compounds. Various hypotheses have been formulated in an effort to define more precisely the role of ethionine as a metabolic antagonist. Simpson, Farber, and Tarver (S), using radioactive methionine, conclude from their experiments that ethionine may inhibit the incorporation of methionine and glycine into liver proteins and prevent the conversion of methionine sulfur to cystine sulfur. In further studies, Levine (9) showed that radioactive ethionine was itself incorporated into tissue protein. In a second series of experiments Stekol and Weiss (lo), from evidence that ethionine can be deethylated in vivo, postulate the synthesis of ethyl analogues which in turn inhibit certain essential transmethylation processes. These two divergent viewpoints, ethionine as a specific methionine antagonist and ethionine as a source of ethyl groups for a series of metabolic analogues, may not be mutually exclusive. Since the reported metabolic action of ethionine is consonant with an inhibition in the oxidation of choline, the effect of ethionine on two enzymes associated with the choline oxidation cycle is the subject of this report. The enzymes chosen for investigation were choline oxidase, the enzyme shown by Dubnoff (11) to be essential for labilizing choline methyl groups through the formation of betaine, and sarcosine oxidase, the enzyme whose substrate sarcosine is an oxidation product of betaine (12). It has been found that the activity of both choline oxidase and sarcosine oxidase is greatly reduced in liver tissue of rats receiving dietary ethionine. To characterize further the ethionine inhibition of these enzymes, one of them, choline oxidase, was studied in a system in vitro. It was found that ethionine could inhibit choline oxidase, but only in a non-dialyzed