Transport and utilization of methionine sulfoxide in the rabbit

Methionine sulfoxide is transported into purified intestinal and renal brush border membrane vesicles from rabbit by an Na+-dependent mechanism and is accumulated inside the vesicles against the concentration gradient. Both in intestine and kidney, the rate of transport is enhanced with increasing concentrations of Na+ in the external medium. Increasing the Na+ gradient reduces the apparent Kt for methionine sulfoxide without causing any change in Vmax. With an outward K+ gradient (vesicle greater than medium), valinomycin stimulates the Na+-gradient-dependent transport of methionine sulfoxide in the kidney, showing the electrogenicity of the transport process. A number of amino acids inhibit methionine sulfoxide transport in both the intestine and kidney. An enzymatic activity capable of reducing methionine sulfoxide to methionine is present in the intestinal mucosa, renal cortex and liver. The activity is highest in renal cortex and lowest in intestine. The methionine sulfoxide-reducing activity is stimulated by NADH, NADPH, glutathione and dithiothreitol and the potency of the stimulation is in the order: dithiothreitol greater than NADPH greater than glutathione greater than NADH.