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Henry Ford Hospital Medical Journal

Abstract

Although several mechanisms have been proposed to explain the transport of hydrophylilc cations across lipoprotein barriers, the nature and identity of the carrier mechanism have not yet been elucidated. The present communication is concerned with a mechanism postulating that monoesterified phosphate on the inositol moiety of phosphatidyl inositol might provide anionic sites that function as cation carriers via a phosphorylation-dephosphorylation cycle, catalyzed in its simplest sequence by phosphatidyl inositol kinase and diphosphoinositide phosphomonoesterase activities. In this context, one might reasonably expect that turnover of monoesterified phosphate in the polyphosphoinositides of isolated renal cortex tubules would be reduced in tubules in which movement of sodium out of, and of potassium into, the tubule cells had been inhibited by ouabain. Contrary to this expectation, turnover of monoesterified phosphate, as evidenced by 32P-orthophosphate incorporation into the monoesterified phosphate of the polyphosphoinositides, was strikingly increased when such tubules from rabbits were incubated in the presence of ouabain. This increase appeared to correlate only with ouabain-inhibited extrusion of sodium from the intracellular compartment of tubule cells, and not at all with the inhibitory effects of ouabain on K+-reaccumulation.

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