PURIFICATION AND CHARACTERIZATION OF L-HISTIDINE AMINOTRANSFERASE FROM NIKKOMYCIN-PRODUCING STREPTOMYCES-TENDAE TU901

Cell extracts of Streptomyces tendae grown in nikkomycin production me dia contained an enzyme (HisAT) that transaminated L-histidine as the sole amino substrate with pyruvate as the amino group acceptor. HisAT was purified about 190-fold from the crude extract of S. tendae. The e nzyme was determined by gel filtration and SDS-PAGE to be a homodimer with a subunit molecular mass of approximately 45 kDa. The aminotransf erase had maximum activity at pH 7.0 and 37-degrees-C. The enzyme was highly specific for L-histidine; pyruvate, 2-oxobutyrate, 2-oxovalerat e and 2-oxocaproate were used as keto acceptors to about the same exte nt. The reaction mechanism was ping-pong. The K(m) values for L-histid ine and pyruvate, determined from Lineweaver-Burk plots, were 25 mm an d 10 mm, respectively. Neither cell extracts of non-producing S. tenda e mutants nor extracts of Streptomyces lividans, a species that does n ot synthesize nikkomycins, showed transaminating activity with a narro w substrate specificity for L-histidine as the amino donor. This stron gly suggests that the formation of HisAT is essential for nikkomycin p roduction.