L-[H-3]ADENOSINE, A NEW METABOLICALLY STABLE ENANTIOMERIC PROBE FOR ADENOSINE TRANSPORT-SYSTEMS IN RAT-BRAIN SYNAPTONEUROSOMES

The stereoenantiomers D-[H-3]adenosine and L-[H-3]adenosine were used to study adenosine accumulation in rat cerebral cortical synaptoneurosomes. L-Adenosine very weakly inhibited rat brain adenosine deaminase (ADA) activity with a K(i) value of 385-mu-M. It did not inhibit rat brain adenosine kinase (AK) activity, nor was it utilized as a substrate for either ADA or AK. The rate constants (fmol/mg of protein/s) for L-[H-3]adenosine accumulation measured in assays where transport was stopped either with inhibitor-stop centrifugation or with rapid filtration methods were 82 +/- 14 and 75 +/- 10, respectively. Using the filtration method, the rates of L-[H-3]adenosine accumulation were not significantly different from the value of 105 +/- 15 fmol/mg of protein/s measured for D-[H-3]adenosine transport. Unlabeled D-adenosine and nitrobenzylthioinosine, both at a concentration of 100-mu-M, reduced the levels and rates of L-[H-3]adenosine accumulation by > 44%. These findings suggest that L-adenosine, a metabolically stable enantiomeric analog, and the naturally occurring D-adenosine are both taken up by rat brain synaptoneurosomes by similar processes, and as such L-adenosine may represent an important new probe with which adenosine uptake may be studied.