Calcium-induced conversion of adenine nucleotides to inosine monophosphate in human red cells.

1. When inosine-fed human red cells are permeabilized to calcium by exposure to the ionophore A23187, progressively larger proportions of the cell population become irreversibly depleted of ATP as calcium influx is increased (Brown & Lew, 1983; García-Sancho & Lew, 1988b). When calcium influx is over 30 mmol/(l cells.h), all cells become ATP depleted and calcium equilibrated (E cells) (García-Sancho & Lew, 1988b). When calcium influx is lower, E cells co-exist with cells able to maintain normal ATP and low calcium contents in vigorous pump-leak balance (B cells). The experiments reported here investigate why calcium-induced ATP depletion of E cells is irreversible. 2. The inosine monophosphate (IMP) content of cells after 30 min of calcium permeabilization increased with the magnitude of the calcium load, roughly in inverse proportion to the fall in ATP. The calcium-induced increase in IMP was confined to the fraction of cells which became osmotically resistant after SCN- treatment (H cells), and which contained the E cells. 3. Cell nucleotides were measured after calcium permeabilization [( A23187]c = 100 mumol/l cells) in substrate-free media with different [Ca2+]o (0-0.5 mM). Calcium entry caused rapid ATP fall, AMP and IMP accumulation, and delayed ADP fall at all [Ca2+]o concentrations. Initial IMP formation increased with [Ca2+]o along a sigmoid saturation-like curve whereas AMP accumulation and ATP fall were maximal at [Ca2+]o = 20 microM and declined at the higher [Ca2+]o. The rate of IMP formation correlated positively with cell ATP and negatively with cell AMP at all [Ca2+]o values. 4. The AMP deaminase activity of red cell lysates was reversibly increased over tenfold by calcium. Half-maximal stimulation was observed at a Ca2+ concentration of about 50 microM. 5. These results suggest that the irreversibility of calcium-induced ATP depletion results from irreversible trapping of the adenine nucleotide as IMP, and help explain the mechanism of E cell formation.