The Participation of Acetyl Phosphate, a Microbial and Host Metabolite, in the Regulation of the Calcium Balance in Mitochondria and Cells

Acetyl phosphate (AcP) is a microbial metabolite acting as a link between cell metabolism and signaling, providing the survival of bacteria in the host. AcP was also identified as an intermediate of pyruvate oxidation in mammalian mitochondria and was found in the human blood in some severe pathologies. The possible contribution of circulating AcP to the maintenance of the physiological or pathological states of the body has not been studied. Since AcP can function as a donor of phosphate groups, we have examined <i>in vitro</i> the influence of AcP on calcium signaling in mitochondria and cells by measuring the membrane potential and the calcium retention capacity of mitochondria by selective electrodes and by assaying the cell calcium signaling by Fura-2AM fluorescent radiometry. AcP was shown to induce a concentration-dependent increase in the mitochondrial resistance to calcium ion loading both in the control and in the presence of ADP. This effect was especially pronounced when mitochondria were incubated in a phosphate-free medium; under these conditions, AcP strongly raised the membrane potential and increased the rate of calcium uptake and the calcium retention capacity several times. Moreover, AcP induced similar changes in human cells when calcium signaling was activated by ATP, to a greater extent in neuroblastoma cells than in astrocytes. In the presence of AcP, a tendency for an increase in the amplitude and a decrease in the continuance of the ATP-induced calcium response was observed. These changes are probably associated with the activation of calcium buffering by mitochondria due to the delivery of phosphate during the hydrolysis of AcP. The results show that AcP is involved in the regulation of the Ca²⁺ balance in cells by activating the accumulation of calcium ions by mitochondria, especially under phosphate deficiency. A shift in calcium signaling mediated by AcP supplementation may be caused by hyperphosphatemia, which is now considered as one of basic contributors to cellular dysfunction and progression of various diseases, including sepsis.