Homoserine deaminase and

A yellow enzyme has been isolated from rat liver in crystalline form which deaminates homoserine giving rise to a-ketobutyrate and ammonia (235). The same preparation catalyzes the cleavage of cystathionine to cysteine. The probable identity of homoserine deaminase and cystathi-onase is proposed. The crystalline enzyme contains pyridoxal phosphate as the prosthetic group. [One mole of enzyme contains 4 moles of pyridoxal phosphate ( 3 ).] No significant quantity of bound heavy metals ions could be detected. The resolution of the enzyme into pyridoxal phosphate and oatalytically inactive apoenzyme and subsequent reconstitution of the active enzyme by combining the two components 2S6a) provides conclusive... 

This pyridoxal-phosphate-dependent enzyme [EC 4.4.1.1] (also referred to as homoserine deaminase, homoserine dehydratase, y-cystathionase, cystine desulfhy-drase, cysteine desulfhydrase, and cystathionase) catalyzes the hydrolysis of cystathionine to produce cysteine, ammonia, and a-ketobutanoate (or, 2-oxobutanoate). 

Overproduction of E (isoleucine) inhibits enzyme E6 (threonine deaminase), and the consequent rise of D (threonine) reduces the rate of production of C (homoserine) via enzyme E3 (homoserine dehydrogenase). The concentration of B (aspartate semialdehyde) rises, and this in turn inhibits Ej (aspartokinase). It is therefore obvious why the control system is called a negative feedback network, or sequential feedback system. 

A purified preparation of homoserine deaminase from rat liver was found by Binkley and Olson" not to attack serine or threonine. The latter finding would appear to rule out the possibility that threonine is an intermediate in reaction 19. Although some activation by AMP and glutathione was observed, neither substance was considered to be essential. The enzyme did not attack n-homoserine nor the lactones of L-homoserine or DL-homoserine. 

Binkley and Okeson purified the enzyme system that cleaves cystathionine and found that neither phosphate nor ATP was required for activity, thus correcting the previous report that ATP was required. In addition to splitting cystathionine, this enzyme preparation also produced H2S from cysteine. The authors suggest that their enzyme may be identical with cysteine desulfhydrase. Binkley also reported that he had been able to synthesize cystathionine enzymatically from homocysteine and serine by a fractionated liver preparation which had been freed from the cystathionine cleavage enzyme, serine dehydrase and homoserine deaminase. The activity of the enzyme synthesizing cystathionine was either inhibited or unaffected by ATP, DPN, AMP, and various metal ions. 

Homoserine deaminase, the enzyme that forms a-ketobutyiic add from homoserine, has been crystallized from rat liver by Matsuo and Greenberg (ffS). The enzyme is colored lemon yellow. It is most interesting that this enzyme also cleaves cystathionine. The role of this enz3rme in the trans-sulfuration reaction is discussed in Chapter 16. 

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