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Lactate Dehydrogenase

XI. EFFECTS OF GUANIDINATION UPON THE PROPERTIES OF THE ENZYME

From the ¹ From the Department of Biochemistry, University of Kentucky Medical Center, Lexington, Kentucky 40506

The H₄ isozyme of bovine lactate dehydrogenase was guanidinated with O-methyl isourea and its properties were compared with those of the native enzyme. Kinetic evidence suggests that coenzyme binding is not affected by guanidination but that the rate of dissociation of substrates is diminished.

Fluorimetric titrations with NADH confirm the kinetic results and show that both oxamate and oxalate are bound more firmly by the complex between guanidinated enzyme and NADH than by the native enzyme·NADH complex.

The changes in binding of inhibitors appear to result from the modification of several amino groups rather than from derivatization of a single essential group. It is proposed that strong interactions between carboxylate and guanidinium ions stabilize the ternary enzyme·coenzyme·inhibitor complex.

Improved methods are presented for calculation of dissociation constants of enzyme·NADH·inhibitor complexes from fluorescence titration curves.

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