The synthetic racemic monoester had the same absorption spectrum. Optically active sodium monoethyl a-acetamidomalonate (experiment L24) showed, in D20, absorption had the same absorption spectrum. bands at 2.95-3.05 7.25 (m), 7.34 (m), 7.55 (w).(w), 5.82 (m), 6.18 (s), 6.78-6.90 (w),The synthetic racemic salt [CONTRIBUTION FROM THEA study of the reaction between pyridoxal-5-phosphate and aminothiols was prompted by the observation that pyridoxal-5-phosphate is released in the form of a complex with cysteine when muscle phsphorylase a is incubatcd in a cysteine buffer at pH 6.8. In aqueous media over a wide range of pH values the 4-formyl group of pyridoxal-5-phosphate reacts with the sulfhydryl group of an addend that contains no amino group to form a highly dissociable thiohemiacetal; it also reacts with the amino group of an addend that contains no sulfhydryl group to form a highly dissociable Schiff base. When both functional groups are present in suitable proximity on the same addend, the product is a relatively stable complex containing a thiazolidine ring. The second order constants describing the rates of complex formation between pyridoxal-5-phosphate and both cysteine and its ethyl ester have been determined as a function of pH and compared with those found for the same reactions with 5-deoxypyridoxal. The results are believed t o bear upon the bonding of pyridoxal-5-phosphate in muscle phosphorylase.

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