Short term reversible modulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in isolated epithelial cells from rat ileum. Regulation of phosphorylation dephosphorylation by bicarbonate.

In ileal epithelial cells isolated through incubation of gut loops in phosphate-buffered saline containing EDTA, 3-hydroxy-3-methylglutaryl coenzyme A reductase was found to exist prdominantly in an active (dephosphorylated) form. However, in cells obtained by scraping the intestine, the enzyme was mostly inactive (phosphorylated) and could be activated several-fold on incubation with a crude phosphatase preparation. Reductase activity in microsomes from ileal epithelial cells could be inactivated extensively by incubation with Mg . ATP in the absence of any exogenous factors. When cells isolated in phosphate-buffered saline were incubated in bicarbonate-containing buffers, there was a time-dependent decrease in the apparent activity of 3-hydroxy-3-methylglutaryl CoA reductase with little change in the total activity of the enzyme. On transferring the cells to a buffer without bicarbonate, the decrease in the apparent activity of the enzyme was fully reversed. There was no change in both the apparent and the total activities of reductase when cells were incubated in buffers lacking bicarbonate. Sterol synthetic rates in isolated intestinal cells were reflective of the fraction of reductase in the active form. These results strongly suggest the operation of a phosphorylation-dephosphorylation mechanism for short term modulation of 3-hydroxy-3-methylglutaryl CoA reductase activity in the intact mucosal cell. The bicarbonate anion appears to function as an activator of this modulation system.