Amino Acid Composition and Properties of Crystalline Lactate Dehydrogenase X from Mouse Testes*

Lactate dehydrogenase (LDH) X has been isolated in crystalline form from mouse testes. Homogeneity of the protein preparation was established by disc gel electrophoresis, analytical ultracentrifugation, and immunochemical analyses. LDH-X is clearly distinct in amino acid composition from LDH-1 and LDH-5 with respect to numbers of residues of leucine, glycine, threonine, and methionine. When 6 to 8 moles of p-hydroxymercuribenzoate are bound per mole of LDH-X substantial enzymic activity remains. A molecular weight of 140,000, comparable to that of beef heart LDH-1, was determined for LDH-X by zone velocity sedimentation in an isokinetic sucrose gradient. When LDH-X is incubated at 65” for 20 min it retains 65 % of its original activity as compared to the virtually complete inactivation of LDH-1 and LDH-5.