BINDING-SITES FOR ALPHA-TRINOSITOL (INOSITOL 1,2,6-TRISPHOSPHATE) IN PORCINE TISSUES - COMPARISON WITH INS(1,4,5)P-3 AND INS(1,3,4,5)P-4-BINDING SITES

1 The molecular mechanism of action of the inositol trisphosphate isom er, alpha-trinositol (Ins(1,2,6)P-3) which has potential therapeutic u se in treatment of inflammation and burn oedema, is still unclear. The refore we have studied binding sites for alpha-trinositol in different tissues. 2 In membranes from pig cerebellum, liver, kidney, heart, an d spleen, the density of specific [H-3]-alpha-trinositol binding sites was maximal at pH 5.0. Cerebellum and spleen showed only one binding site (cerebellum K-D = 9.1 mu M, spleen K-D = 7.3 mu M) In the other t issues, there were a high-affinity site (heart K-D = 70 nM, liver K-D = 790 nM and kidney K-D = 1800 nM), besides a low-affinity site with a K-D ranging between 32 and 120 mu M. In cerebellar membranes, the aff inity and density (107 pmol mg(-1) protein) of alpha-trinositol bindin g sites were not affected by phosphate (0 to 25 mM). 3 Binding of Ins( 1,4,5)P-3 and Ins(1,3,4,5)P-4 to membranes from different porcine tiss ues was also determined. Ins(1,3,4,5)P-4, the isomer stereochemically related to alpha-trinositol, binds with an affinity of 1.2 nM in cereb ellum, but in the other tissues the binding site density was too low t o determine the affinity. With cerebellar membranes heterologous displ acement of [H-3]-Ins(1,3,4,5)P-4 by alpha-trinositol yielded a K-1 of 11 mu M. The Ins(1,4,5)P-3 receptor displayed an affinity of 15 nM in cerebellum and of 5 to 7 nM in the other tissues investigated. 4 The s olubilized Ins(1,3,4,5)P-4 receptor preparation from cerebellum did no t show Ins(1,2,6)P-3 binding. Ins(1,2,6)P-3 binding was found in the p ellet obtained after solubilization of the membranes with the detergen t Brij 58. 5 Thus, in different tissues alpha-trinositol binds to prot eins with different affinity. They are obviously not related to bindin g sites for Ins (1,4,5)P-3 or for Ins(1,3,4,5)P-4. Future experiments have to unravel the identity of the binding protein(s) for alpha-trino sitol.