Bile Acids Stimulate ATP Hydrolysis in the Purified Cholesterol Transporter ABCG5/G8

Abstract

ABCG5 and ABCG8 are half-size ABC transporters that function as heterodimers (ABCG5/G8) to reduce sterol absorption in the intestines and increase sterol excretion from the liver. Previous studies demonstrated that bile acids increased ABCG5/G8 specific cholesterol efflux in cell models. In this study we tested the effects of bile acids on ATP hydrolysis in Pichia pastoris purified ABCG5/G8 and found that they stimulated hydrolysis approximately 20-fold in wild-type ABCG5/G8 but not in a hydrolysis-deficient mutant. Nonconjugated cholate supported the highest ATPase activity in ABCG5/G8 (256 +/- 9 nmol min(-1) mg(-1)). ATP hydrolysis was also stimulated by other conjugated bile acids and a mixture of bile acids resembling human bile with activities ranging from 129 +/- 4 to 147 +/- 14 nmol min(-1) mg(-1). The kinetic parameters, inhibitor profiles, and lipid requirements of bile acid stimulated ATP hydrolysis were characterized. Cholate-stimulated ATP hydrolysis was maximal at concentrations of >or=10 mM MgATP and had a relatively high K(M) (MgATP) of approximately 1 mM. Orthovanadate, BeFx, and AlFx effectively inhibited ABCG5/G8 at concentrations of 1 mM. Various lipid mixtures supported bile acid-stimulated ATP hydrolysis, which increased when cholesterol was present. The data demonstrate that bile acids together with lipids and cholesterol increase ATP hydrolysis in purified ABCG5/G8. Bile acids may promote an active conformation of purified ABCG5/G8 either by global stabilization of the transporter or by binding to a specific site on ABCG5/G8.