ISOZYMES OF LACTIC DEHYDROGENASE; THEIR ALTERATIONS IN ARTHRITIC SYNOVIAL FLUID AND SERA*

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Abstract
Synovial fluid from 12 patients with various types of arthritis contained 1.4 to 56 times the LDH activity of an equal volume of their serum. Total serum LDH activity was not usually elevated in these patients, but their serum isozymes were generally altered and revealed a significantly higher percentage of isozyme 5 (10.6 [plus or minus] 3.8 per cent) than did normal serum (0.3 per cent). Joint fluids contained high activities of isozyme 5. Since the isozyme patterns of leukocytes and of synovial tissue both resemble that of joint fluid, the elevated LDH activity of joint fluid may have its source either in the white cell, or the synovial membrane, or both. Localization on starch gels of the LDH isozymes from homogenates of various human tissues showed that each tissue possessed four anodal isozymes and at least one cathodal isozyme. Differences among tissues consisted primarily in the distribution of the total LDH activity among isozymes. Liver, skeletal muscle, skin, and leukocytes exhibited most activity in the cathodal bands, whereas most of the LDH activity of heart, kidney, and erythrocytes was concentrated in the two most rapidly migrating anodal bands. Lung LDH activity was equally divided among the five isozymes. The only cell studied lacking isozyme 5 was the mature erythrocyte. The five human isozymes were isolated and found to behave identically when subjected to ultracentrifugation in a sucrose density gradient. Since most human tissues contain the same number of LDH isozymes and differ only in distribution of total activity among them, localization of pathology to a single organ solely by analysis of serum LDH isozymes may prove misleading. Isozyme 5, previously regarded as an index of liver damage, is also elevated in arthritic synovial fluid and sera.