Resolution of D-serine dehydratase by cysteine. An analytical treatment.

A general method is presented for analysis of the resolution of pyridoxal-P-requiring enzymes by carbonyl reagents. The method is useful for accurately determining the very small equilibrium constants (KP) which characterize the dissociation of cofactor from many pyridoxal-P-requiring enzymes. The analysis also establishes the minimum number and relative stabilities of distinct enzymic species involved in the resolution process. Analysis of the resolution of D-serine dehydratase by L-and D-cysteine resulted in the establishment of an enzyme bound thiazolidine derivative as an intermediate in the pathway for resolution. The over-all equilibrium constant (KR) for the reaction, D-serine dehydratase + cystein in equilibrium KR thiazolidine derivative +D-serine apodehydratase was determined. At pH 7.80, T/2 0.33, 25 degrees, KR equal to 1.08 times 10-minus 3. A value of 7.0 nM for the equilibrium constant for the dissociation of D-serine dehydratase to apoenzyme and free pyridoxal-P was determined from the ratio KR/KT, where KT is the equilibrium constant for the formation of a thiazolidine derivative from free pyridoxal-P and cysteine. An estimate of 14 nM for KP was also obtained from partial resolution of D-serine dehydratase by high dilution. The difficulties associated with this direct determination of KP from the dependence on the enzyme concentration of the activity of very dilute solutions of enzyme are discussed.