Partial characterization of insulin receptors from rat myocytes

Isolated myocytes were prepared from the adult rat heart and characterized for viability. The myocytes were exposed to 125I-insulin, and the 125I-insulin-receptor complex was extracted with 1% Triton X-102 and then applied to a DEAE-Sephacel column. When the chromatography were applied to a Sepharose CL-6B column, a 140,000-dalton complex with high specific radioactivity was found. Alternatively, when myocyte insulin receptors were first extracted with 1% Triton X-102 without prior exposure to 125I-insulin and then applied to a DEAE-Sephacel column, three peak protein fractions were obtained. They were treated separately with 125I-insulin and the 125I-insulin-protein complexes were covalently cross-linked with disuccinimidyl suberate. The cross-linked samples were applied to a Sepharose CL-6B column and the radioactive protein fractions were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The first and second peak samples from the ion exchange chromatography yielded complexes of low specific radioactivity, which appear to be formed by nonspecific random binding of 125I-insulin to the solubilized membrane proteins. In contrast, Sepharose CL-6B gel filtration of the cross-linked sample from the third peak fractions gave a major highly radioactive 125I-insulin-receptor complex with a molecular weight of 370,000 and a minor complex of low radioactivity with a molecular weight of 140,000. Upon sodium dodecyl sulfate-gel electrophoresis, the 370,000-dalton complex was dissociated to 130,000- and 82,000-dalton components and the 140,000-dalton complex was dissociated to a 47,000-dalton component.

Inhibition of S37 ascites cell amino acid transport systems by alpha-chloromethylketone analogs

Alanine chloromethylketone and leucine chloromethylketone were synthesized and their effects on amino acid transport in sarcoma 37 mirone ascites tumor (S37) cells were studied. Alanine chloromethylketone preincubation weakly inhibited system A. Leucine chloromethylketone preincubation strongly inhibited both amino acid transport systems L and A. Leucine chloromethylketone was also a competitive inhibitor of leucine transport. Labeled leucine chloromethylketone was concentrated by S37 cells. Leucine chloromethylketone preincubation inhibition was concentration dependent and partial protection of transport was afforded by leucine. Steady-state retention of amino acids was decreased more than the initial velocity of transport by leucine chloromethylketone preincubation. Glutathione was also depleted. Labeled leucine chloromethylketone was incorporated in a plasma membrane protein fraction comigrating on a DEAE-cellulose column (DE52) with gamma-glutamyltranspeptidase activity. There was a modest increase in vital staining after treatment of S37 cells with leucine chloromethylketone, and glucose uptake was also inhibited. Whilst several effects occur during treatment of S37 cells with leucine chloromethylketone, it is suggested than one prominent effect is alkylation of amino acid transport system components.