Dissimilation of Methionine by a Demethiolase of Aspergillus Species1
AUTOR(ES)
Ruiz-Herrera, José
RESUMO
Enzyme preparations obtained from the mycelium of Aspergillus species broke down methionine by co-dissimilation. The deaminase and demethiolase activities of crude extracts were increased 100-fold by precipitation with (NH4)2SO4 and column chromatography on diethylaminoethyl cellulose. The enzyme acted on d-methionine but not on l-methionine. The enzyme was labile: it was inactivated by oxygen and ascorbic acid but ethylenediaminetetraacetic acid and mercaptoethanol preserved its activity. Enzyme activity decreased even at 4 and −30 C and was lost rapidly above 45 C. It was most rapid at 35 C and at pH 8.0 to 9.0. For the following reasons, it was concluded that deamination and demethiolation of methionine were effected by the same enzyme: both activities increased equally at each stage of purification; ammonia, methanethiol, and α-keto butyric acid were formed in amounts equivalent to the amount of methionine dissimilated; the Km and optimal pH for formation of both keto acid and methanethiol were the same; both activities remained in the same fractions that were separated by electrophoresis and the activities were equivalent. The purified enzyme demethiolated α-keto methionine and α-hydroxy methionine and split the sulfur linkage of ethionine but did not cleave cystathionine. Few amino acids were deaminated. The enzyme was sensitive to some carbonyl and sulfhydryl reagents and was relatively insensitive to heavy metals other than Hg++. The Km was 1.3 × 10−3 to 1.5 × 10−3m at pH 7.0. No requirement for cofactors was noted, and attempts to dissociate the enzyme, including dialysis with hydroxylamine, were unsuccessful.
