The interconversion of serine and glycine: some further properties of the enzyme system

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Abstract
The rate of synthesis of serine from glycine and formaldehyde by a partially purified enzyme from rabbit liver was proportional to enzyme concentration and time of incubation. The reaction ultimately reached an equilibrium corresponding to a value of the constant (glycine) (formaldehyde)/ (serine) of approximately 5.8 m[image]. The relation between glycine concentration and reaction velocity conformed to the requirements of the simple Michaelis-Menten equation, giving a Km of 2.8 x 10-2 [image]. The optimum formaldehyde concentration was 0.01 [image], the reaction velocity diminishing at higher concentrations. Hydrogenated pteroylglutamic acid (PGA), Nl0-formylPGA, pteroyltriglutamate and pteroylhepta-glutamate were the only pteridines active as intermediates among those tested. The 2-amino and 4-hydroxy groups, the hydrogenated pyrazine ring and one or more glutamic acid residues are essential for activity. Leucovorin, citrovorum factor and anhydroleucovorin A are all inactive. Dihydro-2-deaminoPGA, but not tetrahydro-4-amino-4-deoxyPGA, is an inhibitor of serine synthesis with tetrahydroPGA as coenzyme. The relations between reaction velocity and the concentration of dihydroPGA, tetrahydroPGA and tetrahydropteroyltriglutamate respectively conform to the requirements for simple Michaelis-Menten equations giving Km values of 2.9 x 10-4 [image], 1.8 x 10-4 [image] and 8.3 x 10-5 [image] respectively. The reaction productsfrom incubation of enzyme and tetrahydroPGA with serine or formaldehyde do not contain any new pteridine detectable by chromatography, and are inactivated by aeration at about the same rate as a simple mixture of solutions of tetrahydroPGA and formaldehyde. Incubation of enzyme, dihydroPGA and (3-cl4) serine resulted in only very slight incorporation of radiocarbon into pteridines. The nature of the pteridine intermediate of enzymic serine synthesis is discussed in terms of these results.