Characterization of betaine efflux from rat liver mitochondria

In order to investigate the control of endogenous betaine supply to the cytoplasmic enzyme betaine-homocysteine methyltransferase, it was necessary to understand how betaine synthesized within the mitochondrial matrix is transported across the mitochondrial inner membrane. Mitochondria were loaded with radiolabelled betaine and efflux was measured in a medium at physiological ionic strength. Efflux of radiolabelled betaine occurred continuously with time. The efflux rate was unaffected by the presence or absence of a source of energy except at high membrane potentials, where betaine efflux rate increased 2-3-fold. Titration of the membrane potential demonstrated a non-ohmic relationship between betaine efflux rate and membrane potential. The rate of betaine efflux was proportional to the matrix betaine concentration up to 9 mM. Efflux was unaffected by addition of analogues of betaine and known mitochondrial transport inhibitors. N-Ethylmaleimide did inhibit efflux by 50%, but evidence suggested that the effect was non-specific. The lack of saturability or other evidence for a transport system suggests that betaine escapes from mitochondria by simple diffusion. The relative diffusion rates of glycine, sarcosine, dimethylglycine and betaine suggest that increasing the degree of N-methylation lowers diffusion rate.