Dependence of mitochondrial coenzyme A uptake on the membrane electrical gradient

A G Tahiliani

doi:10.1016/s0021-9258(18)51483-6

Abstract

Coenzyme A (CoA) transport was studied in isolated rat heart mitochondria. Uptake of CoA was assayed by determining [3H]CoA associated with mitochondria under various conditions. Various oxidizable substrates including alpha-ketoglutarate, succinate, or malate stimulated CoA uptake. The membrane proton (delta pH) and electrical (delta psi) gradients, which dissipated with time in the absence of substrate, were maintained at their initial levels throughout the incubation in the presence of substrate. Addition of phosphate caused a concentration-dependent decrease of both delta pH and CoA uptake. Nigericin also dissipated the proton gradient and prevented CoA uptake. Valinomycin also prevented CoA uptake into mitochondria. Although the proton gradient was unaffected, the electrical gradient was completely abolished in the presence of valinomycin. Addition of 5 mM phosphate 10 min after the start of incubation prevented further uptake of CoA into mitochondria. A rapid dissipation of the proton gradient upon addition of phosphate was observed. Addition of nigericin or valinomycin 10 min after the start of incubation also resulted in no further uptake of CoA into with mitochondria; valinomycin caused an apparent efflux of CoA from mitochondria. Uptake was found to be sensitive to external pH displaying a pH optimum at pHext 8.0. Although nigericin significantly inhibited CoA uptake over the pHext range of 6.75-8, maximal transport was observed around pHext 8.0-8.25. Valinomycin, on the other hand, abolished transport over the entire pH range. The results suggest that mitochondrial CoA transport is determined by the membrane electrical gradient. The apparent dependence of CoA uptake on an intact membrane pH gradient is probably the result of modulation of CoA transport by matrix pH.

Highlights

Dition of 5 mM phosphate 10 min after the start of transporters which involve movement of charges across the incubation prevented further uptake of Coenzyme A (CoA) into mi- membrane are operated by the membrane electrical gradient
The results suggest thatand themedium used contained 0.225 M mannitol, 0.75 M sucrose, 10 mitochondrial CoA transport is determinedbythe mM Tris, and0.1 mM EDTA
CoA uptake was significantly lower in the presence of 1 mM phosphate, and the inhibition was more pronounced with 5 mM phosphate (Fig. 2 A )

Summary

CoAinTransport Cardiac Mitochondria

CoA uptake was significantly lower in the presence of 1 mM phosphate, and the inhibition was more pronounced with 5 mM phosphate (Fig. 2 A ). Nigericin, which exchanges H+ for K+ [24], dissipated the membrane proton gradient (from 1.1 pH units to 0.3 pH units) without affecting the electrical gradient. CoA uptake was abolished in the presence of nigericin (Fig. 2B). Than 40 mM K+, initial values in nigericin-containing incubations were lower, and they remained low until the end of incubation (Table I, Fig. 3, inset). The decrease in CoA uptake a t lower K+ concentrations could be attributed to the initial fall in ApH. It is not clear why CoAuptake decreased with increasing K+ concentrations in the presence of nigericin in the lower concentration range of K+ (0-20 mM) despite similar initial and final ApH values (Table I).

CoA Transporti n Cardiac Mitochondria

DISCUSSION

Findings

Matrix pH