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  • Title: Functional and structural differences in human and rat-derived insulin receptors: characterization of the beta-subunit kinase activity.
    Author: Brillon DJ, Henry RR, Klein HH, Olefsky JM, Freidenberg GR.
    Journal: Endocrinology; 1988 Oct; 123(4):1837-47. PubMed ID: 2843349.
    Abstract:
    We studied the kinase activity of partially purified insulin receptor preparations from various rat and human tissues. Time courses for in vitro autophosphorylation were determined, and times to reach half-maximal (t1/2 max) and maximal (tmax) 32P incorporation were compared. Insulin receptors from rat muscle, liver, and fat had a t1/2 max of 7-10 min and a tmax of 60 min; human-derived insulin receptors had a t1/2 max in excess of 30 min and a tmax of 120 min. A spectrum of autophosphorylation time courses was present in human tissues; placenta-derived receptors exhibited a t1/2 max of 13 min while receptors from monocytes and fibroblasts had t1/2 max values of 60 and 80 min, respectively. The ATP Km for autophosphorylation of human-derived receptors was 5-fold greater than that of rat-derived receptors (266 +/- 27 vs. 48 +/- 8 microM, respectively). In contrast, when the receptors were first maximally prephosphorylated, the ATP Km values for substrate phosphorylation of human- and rat-derived receptors were equivalent (12.5 and 11.4 microM). Kact values for Mn were comparable in both human- and rat-derived adipocyte receptors. In addition to the functional differences between species, the apparent mol wt of the beta-subunit of rat-derived receptors (96,000) was consistently greater than that of human-derived receptor beta-subunits (93,000). In contrast to these in vitro findings, the ability of insulin to stimulate receptor kinase activity in isolated adipocytes was rapid, with a maximal effect by seconds. This was comparable for both rat and human tissues, suggesting that the in vitro autophosphorylation differences may not govern kinase activity in vivo.
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