FASTING ENHANCES TAURINE TRANSPORT BY RAT-LIVER PLASMA-MEMBRANE VESICLES

Hepatic taurine stores are maintained by biosynthesis from the sulfur-containing amino acids, methionine and cysteine, and by uptake via a Na+- and Cl--dependent transport system, which is specific for beta-amino acids. We hypothesized that liver stores of taurine are maintained by enhanced hepatic transport during fasting when dietary sources for taurine and its precursors are diminished. Liver plasma membrane vesicles, enriched for the basolateral domain, were prepared from adult male rats fasted for 72 h and from control rats. The maximum velocity for Na+-dependent taurine uptake was twofold greater for the fasted group compared with the control group (0.87 +/- 0.09 vs. 0.31 +/- 0.03 nmol.mg protein(-1).min(-1)). The apparent Michaelis constant for taurine was also greater for fasted compared with control (154.0 +/- 0.5 vs. 80.0 +/- 2.0 mu M) gamma-Aminobutyric acid, but not alanine or glutamine, abolished the effect of fasting on hepatic taurine transport. To determine the effect of fasting independent of changes in the lipid microenvironment, taurine uptake was measured in proteoliposomes reconstituted by inserting detergent-solubilized membrane proteins into asolectin vesicles. Taurine uptake by proteoliposomes reconstituted from membranes prepared from the fasted group was significantly greater than from the control group. We conclude that Na+-dependent taurine transport is enhanced in Liver plasma membranes prepared from fasted rats. Our findings imply that enhanced taurine uptake with fasting is due to either an increased number of functional carriers or activation of existing transporters.