Candida L-norleucine,leucine:2-oxoglutarate aminotransferase. Purification and properties

A new enzyme which catalyzes the transamination of L-norleucine (2-aminohexanoic acid) and L-leucine with 2-oxoglutarate was purified to homogeneity from cells of Candida guilliermondii var. membranaefaciens. The relative molecular mass determined by gel filtration was estimated to be close to 100,000. The transaminase behaved as a dimer which consists of two subunits identical in molecular mass (Mr 51,000). The enzyme has a maximum activity in the pH range of 8.0-8.5 and at 55 degrees C. 2-Oxoglutarate, and to a lesser extent pyridoxal 5'-phosphate, were effective protecting agents against increasing temperature. The enzyme exhibits absorption maximum at 330 nm and 410 nm. L-Norleucine, and L-leucine to a lesser extent, are the best amino donors with 2-oxoglutarate as amino acceptor. The Km values for L-norleucine, L-leucine and 2-oxoglutarate determined from the Lineweaver-Burk plot were 1.8 mM, 6.6 mM and 2.0 mM respectively. A ping-pong bi-bi mechanism of inhibition with alternative substrates is found when the enzyme is in the presence of both L-norleucine and L-leucine. The inhibitory effect of various amino acid analogs on the transamination reaction between L-norleucine and 2-oxoglutarate was studied and Ki values were determined.