Acetyl coenzyme A carboxylase and fatty acid synthetase activities were studied to determine the biochemical basis of the markedly impaired capacity of fat cells from spontaneously obese, old rats to convert glucose to fatty acids relative to cells from lean, young rats. Michaelis constants for the substrates of both enzymes were similar in large and small adipocyte homogenates. In contrast, Vmax values were over 80% less in homogenates from large relative to small cells on a per cell basis. Long-term dialysis or the presence of albumin during the assays failed to restore the activities of these enzymes in homogenates of large fat cells. The combination of equal volumes of homogenates from the two cell types resulted in carboxylase and synthetase activities intermediate between activities found in the two homogenates alone. Therefore, the presence of endogenous allosteric inhibitors does not appear to account for the markedly blunted fatty acid synthesis enzyme activities in large fat cells. These results suggest that the fatty acid synthesis impairment, which is a primary defect in the insulin resistance of the large cells, is at least partly due to diminished cellular contents of acetyl coenzyme A carboxylase and fatty acid synthetase.

In the presence of 5mM glucose insulin only modestly activated rates of glucose uptake by rat epitrochlearis muscles while the rate of glycogen formation from D(U-14C) glucose was markedly stimulated by the hormone. No effect of insulin on lactate output could be detected under these conditions. The activation of labeled glycogen formation by insulin occurred in a dose-dependent manner and a maximal effect was noted at hormone concentrations greater than 4 mU/ml. However, glycogen accumulation by epitrochlearis muscles obtained from old, spontaneously obese rats was activated by only 38 +/- 15% by a supermaximal insulin concentration (200 mU/ml) compared to a 123 +/- 43% stimulation observed in muscles from small rats. This impaired responsiveness to the hormone could not be explained by inhibition of the glycogen synthetase system by increased amounts of endogenous glycogen in the epitrochlearis muscle of spontaneously obese rats. The magnitude of this resistance greatly exceeds the modest reduction in insulin receptor number reported for msucle membranes in obese rats which suggests that other defective cellular components contribute to this syndrome.