Characterizations of two forms of inositol 1,4,5-trisphosphate receptor in rat liver

Abstract

Two binding sites for [ 32P]myo-inositol 1,4,5-triphosphate (Ins(1,4,5)P 3) were detected in a crude particulate fraction prepared from rat liver homogenate and in permeabilized hepatocytes. The same high- and low-affinity sites with K D s of 1.8–2.6 nM and 35–71 nM, respectively, were detected in subcellular fractions enriched in plasma membranes, mitochondria and microsomes, with relative proportions close to those found in the crude membrane function. The order of potency of three inositol phosphates in inhibiting [ 32P]Ins(1,4,5)P 3 binding to the two sites, i.e. Ins(1,4,5)P s > Ins (2,4,5(P 3] > Ins(1,3,4,5)P 4, and the inhibition by heparin, strongly suggest that neither of the binding sites reflected components due to the 3-kinase or the 5-phosphatase. A close correlation was observed between the dose-response curves for Ca 2+ release by Ins(1,4,5)P 3 and Ins(2,4,5)P 3 and the occupancy of the low-affinity binding site by these agonists. These results support the view that the two [ 32P]Ins(1,4,5)P 3 binding sites are two forms of the same receptor.