Isolation and Characterization of Cytoplasmic l‐Glycerol‐3‐Phosphate Dehydrogenase from Rabbit‐Renal‐Adipose Tissue and Its Comparison with the Skeletal‐Muscle Enzyme

Cytoplasmic NAD‐linked l‐glycerol‐3‐phosphate dehydrogenase was isolated from rabbit renal adipose tissue in a preparation that was apparently homogeneous by preparative isoelectric focusing and polyacrylamide‐disc‐gel electrophoresis. The adipose tissue and skeletal muscle enzymes were indistinguishable by disc‐gel electrophoresis, immunological comparisons, apparent Km, values for substrates and coenzymes, isoelectric point in preparative isoelectric‐focusing runs, and apparent molecular weight and Stokes radius on polyacrylamide‐gel‐filtration columns. The ultraviolet spectrum of the adipose tissue enzyme was identical to that of the muscle enzyme including the broad maximum at 275 nm indicative of the non‐protein component characteristic of the muscle enzyme. Crude homogenates of rabbit adipose tissue and muscle electro‐focused in polyacrylamide gels and stained for glycerol‐3‐P dehydrogenase activity showed four major zones present in slightly different proportions in the two homogenates, but fully superimposable in runs on mixed tissue homogenates. Comparison of amino acid composition of the adipose tissue enzyme with analyses of the muscle done in five different laboratories showed that the adipose tissue enzyme cannot be distinguished from the muscle enzyme by this criterion. Similar comparison of the adipose tissue and muscle enzymes with the liver enzyme suggests that the liver enzyme is not identical to the muscle‐adipose form of the enzyme. Possible similarity in physiological role for the muscle‐adipose form of glycerol‐3‐P dehydrogenase in these two tissues and a rationale for their absence in rapidly growing cancer cells is suggested.