A note on the specificity of the enzymatic cleavage of thioethers.

The specificity of the cleavage of thioether compounds related to cysteine by an enzyme from liver tissue of rats was described in an earlier report (I), and experiments as to the availability of thioether derivatives of cysteine and homocysteine for growth have been reported from this laboratory (2). This report is concerned with a brief description of studies of the cleavage of t.he thioether derivatives related to homocysteine by the enzyme of liver tissue. The methods and the preparation of enzyme were described in the previous report (1). The results of the experiments with the compounds are given in detail in Table I. It is to be noted that the amount of enzyme was about 100 times greater than that employed with the derivatives of cysteine. Homocysteine was cleaved by the enzyme with the formation of hydrogen sulfide. Methionine was cleaved with the formation of methyl mercaptan, and ethyl mercaptan was identified as a product of cleavage of ethionine. The S-carboxyalkyl derivatives were cleaved at an increasing rate as the length of the carbon chain was increased; this behavior is in contrast to similar derivatives of cysteine. Since ammonia and sulfhydryl compounds were formed in more or less equal amounts, it is probable that sulfur-containing amino acids were not a product of the cleavage of any of these derivatives; this observation is in agreement with the results of the study of the availability for growth (2). In order to gain information as to whether the order of the rate of cleavage of the compounds corresponded to the order of the rate of combination with the enzyme, the inhibition of the cleavage of L-allocystathionine was tested (Table II). All the compounds were found to be competitive inhibitors of the cleavage in much the same order as the rate of c1eavage.l