The significance of lactate dehydrogenase isozymes in abnormal human skeletal muscle

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Abstract
Selective decrease of LDH-5 (the slowest-moving LDH isozyme) was found on the electrophoretic pattern of skeletal muscle from patients with a wide variety of neuromuscular diseases. Sections of human muscles deficient in LDH-5 were characterized by (1) a tendency toward uniform LDH activity of the fiber types and (2) a weakness of LDH activity in the intermyofibrillar networks of type I fibers and in the stellate formations of type II fibers. These were the intracellular sites previously identified with the slow-moving LDH isozymes in normal muscle. The selective weakness of these sites in the muscles lacking LDH-5 confirmed this localization. The mechanism responsible for selective LDH-5 deficiency is unknown, but this study indicates that a hereditary genetic defect in isozyme production is not the only factor capable of causing this phenomenon. The vulnerability of LDH-5 may be related to its possible physiological role as a glycolytic enzyme.