RESUMEN
Glutamate synthase, isolated in apparently homogeneous form (Mr approximately 265,000) from Saccharomyces cerevisiae after 7500-fold purification, is markedly inhibited by homocysteine sulfonamide. Inhibitions competitive with respect to L-glutamine; the apparent Ki value calculated for L-homocysteine sulfonamide is 3.6 microM; the apparent Km value for L-glutamine is 280 microM. The very high affinity of the inhibitor for the enzyme, as well as structural considerations, suggest that homocysteine sulfonamide is a transition state inhibitor. The previously reported growth inhibitory properties of homocysteine sulfonamide (Reisner, D. B. (1958) J. Am. Chem. soc. 78, 5102-5104) may be due, at least in part, to inhibition of glutamate synthase. L-Methionine sulfone is also a potent competitive inhibitor, whereas L-albizziin, L-methionine-SR-sulfoximine, and L-methionine-SR-sulfoxide are much less effective inhibitors. S. cerevisiae glutamate synthase, which is composed of two dissimilar subunits, uses NADH exclusively, and exhibits low but definite activity when NH3 is substituted for glutamine.