Feedback Inhibition by Methionine and S-Adenosylmethionine,and Desensitization of Homoserine O-Acetyltransferase in Brevibacterium flavum

: Homoserine O-acetyltransferase [EC 2.3.1.31] partially purified from Brevibacterium flavum was found to be specifically inhibited by the metabolic end products methionine and S-adenosylmethionine only when the enzymatic reaction was performed in the presence of cysteine or dithiothreitol, or after the preincubation of the enzyme with either of the sulfhydryl compounds. p-Hydroxymercuribenzoate desensitized the enzyme to inhibition. Concentrations of methionine and S-adenosylmethionine giving 50% inhibition were 4.8 and 0.26 mM, respectively, and 0.5 mM S-adenosylmethionine showed almost complete inhibition. No synergistic action by the two inhibitors was found. Optimum pHs were 7.5 and 8.5 for the inhibition by methionine and S-adenosylmethionine, respectively. The inhibitions by the former and the latter were of mixed type and non-competitive respectively, with respect to both substrates, homoserine and acetyl-CoA. Plots of the reaction rate against concentration of the inhibitors were sigmoidal, indicating the presence of co-operativity. N-Formylmethionine, alpha-methylmethionine, trifluoromethionine, selenomethionine, ethionine or S-adenosylhomocysteine inhibited the enzyme to almost the same extent as methionine or S-adenosylmethionine. The enzyme irreversibly lost sensitivity to inhibition during extraction or storage. Sensitivity was retained by the addition of cysteine, dithiothreitol, homoserine (substate), or glycerol.