ATP STIMULATES LYSOSOMAL SULFATE TRANSPORT AT NEUTRAL PH - EVIDENCE FOR PHOSPHORYLATION OF THE LYSOSOMAL SULFATE CARRIER

ATP markedly stimulated sulphate uptake by rat liver lysosomes that ha d been treated with N-ethylmaleimide to block the effects of the lysos omal proton-translocating ATPase (H+-ATPase). Maximal stimulation requ ired millimolar concentrations of ATP and neutral buffer pH. ATP-stimu lated transport exhibited saturation kinetics with a K-m of 175 mu M, identical with the K-m for lysosomal sulphate uptake at pH 5.0, a proc ess that does not require ATP. The requirement for ATP was specific: o ther nucleotides such as AMP, ADP, CTP, GTP, ITP and UTP failed to sti mulate transport. Adenosine 5'-[beta,gamma-imido]triphosphate, the non -hydrolysable analogue of ATP, also failed to stimulate sulphate uptak e, suggesting a requirement for ATP hydrolysis. Lysosomal pH, membrane potential and glucose transport were unchanged by the presence of ATP under the experimental conditions, consistent with a direct effect of ATP on the sulphate transporter. Exposure of lysosomes to protein kin ase A and protein kinase C inhibitors did not alter the stimulation of sulphate transport by ATP. The lysosomal sulphate transport protein m ight be subject to regulation by a phosphorylation pathway that is not dependent on protein kinase A or protein kinase C.