Interactions of Anaerobic Propionate Formation and Acid-Base Status inArenicola marina:An Analysis of Propionyl-CoA Carboxylase

Abstract

The contribution of propionyl-CoA carboxylase (PCC) to the control of anaerobic metabolism by acid-base parameters (pH, Pco₂, and [HCO₃⁻]) was investigated with a purified enzyme preparation and isolated mitochondria from the body wall musculature of Arenicola marina. The enzyme catalyzes the rate-limiting step in anaerobic propionate formation, namely, the carboxylation of methylmalonyl-CoA with concomitant formation of ATP and base equivalents (= HCO₃⁻). Propionyl-CoA carboxylase is likely not saturated with its substrates methylmalonyl-CoA, ADP, and Pi under in vivo conditions, and propionate formation is therefore activated by a decreasing energy charge of the cell (i. e., increasing ADP and Pi concentrations). The effects of the individual acid-base parameters pH, Pco₂, and [HCO₃⁻] on PCC activity have been determined. Stimulation of PCC by both high proton and low bicarbonate concentrations reflects an amplified control of propionate formation by the intracellular acid-base status. Nonrespiratory acidosis enhances the rate of decarboxylation of methylmalonyl-CoA, leading to a release of base equivalents. This mechanism has a strong stabilizing effect on the intracellular pH during long-term anaerobiosis. Without bicarbonate production by PCC, an additional pH drop of about 0. 03 pH units per hour of anaerobiosis would be observed in A. marina. Our data support the hypothesis that, besides ionic transport mechanisms, metabolism itself contributes to cellular pH regulation.