Binding and degradation of insulin by plasma membranes from bovine liver isolated by a large scale preparation.

With the large-scale preparation described, as much as 1 kg of bovine liver can be processed, giving a yield of more than 1 g plasma membrane protein. From analytical and morphological criteria the plasma membrane fraction isolated mainly derives from bile-canalicular and contiguous areas of the hepatocytes. The insulin binding activity is quite similar to insulin receptors in other cell systems and membrane preparations. Insulin-degrading activity is very low in the isolated plasma fraction. Most of degrading activity is located in a microsomal membrane fraction. Nevertheless the Km and the pH dependence of the insulin-degrading activity in both fractions are nearly identical. From these studies we conclude that binding and degradation of insulin are two independent processes located on different cell organelles.