On the functional significance of LDH X.

We have analyzed the catalytic properties and subcellular distribution of the lactate dehydrogenase isozyme specific for testis and sperm (LDH X or C4). LDHX catalyzes nicotinamide adenine dinucleotide (NAD)-linked interconversion alpha-ketoacid in equilibrium alpha-hydroxyacid with a variety of compounds. Isozyme X is located in the cytosol and in the matrix of a special type of mitochondria present in spermatoza. We have postulated an additional shuttle which can transfer hydrogen from cytosolic NADH to the mitochondrion by the redox couple alpha-hydroxyacid/alpha-ketoacid. The operation of this system has been demonstrated in vitro. LDH X may play a role in processes which provide energy to spermatozoa.