INTERACTIONS OF ANAEROBIC PROPIONATE FORMATION AND ACID-BASE STATUS IN ARENICOLA-MARINA - AN ANALYSIS OF PROPIONYL-COA CARBOXYLASE

The contribution of propionyl-CoA carboxylase (PCC) to the control of anaerobic metabolism by acid-base parameters (pH, Pco(2), and [HCO3-]) was investigated with a purified enzyme preparation and isolated mito chondria from the body wall musculature of Arenicola marina. The enzym e catalyzes the rate-limiting step in anaerobic propionate formation, namely, the carboxylation of methylmalonyl-CoA with concomitant format ion of ATP and base equivalents (= HCO3-). Propionyl-CoA carboxylase i s likely not saturated with its substrates methylmalonyl-CoA, ADP, and P-i under in vivo conditions, and propionate formation is therefore a ctivated by a decreasing energy charge of the cell (i, e., increasing ADP and P-i concentrations). The effects of the individual acid-base p arameters pH, Pco(2), and [HCO3-] on PCC activity have been determined . Stimulation of PCC by both high proton and low bicarbonate concentra tions reflects an amplified control of propionate formation by the int racellular 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 in tracellular pH during long-term anaerobiosis. Without bicarbonate prod uction by PCC, an additional pH drop of about 0.03 pH units per hour o f anaerobiosis would be observed in A. marina. Our data support the hy pothesis that, besides ionic transport mechanisms metabolism itself co ntributes to cellular pH regulation.