Isolation and properties of lactate dehydrogenase isozyme X from swiss mice

Abstract Lactate dehydrogenase X, a sixth LDH isozyme in testes and mature sperm, has been purified to homogeneity from adult mouse testes. The isozyme has a molecular weight of 139,000 ± 6000. Dissociation of the molecule with 6 m guanidine produced a homogeneous material with mol wt 37,700 ± 800. Pyruvate at 37 ° was reduced optimally at about pH 7.25 and a concentration of 0.25 m m . Alpha-ketobutyrate has an optimum pH of 7.75 and concentration of 0.5 m m for reduction with a rate about 50% greater than that of pyruvate. Alpha-ketoglutarate has an optimum pH of about 5.5 and is reduced at only about one-fourth the rate of pyruvate. Pyruvate reduction is considerably inhibited by 2.5 m m lactate. Alpha-hydroxybutyrate and α-hydroxyvalerate are oxidized at about the same rate and under similar optimum conditions as lactate. Alpha-hydroxyglutarate is oxidized at only about one-twentieth the rate of lactate.