Uptake and hydrolysis of glycylglutamine at the blood-brain barrier.

Using an organ balance technique in dogs, we recently found that liver, skeletal muscle, kidney, and intestine participate in clearance of glycylglutamine from plasma. The purpose of the present study was to investigate whether brain does the same. The study of arteriovenous differences of glycylglutamine across brain, during continuous infusion of this dipeptide (12 mumol.min-1.kg-1) in dogs, showed an arteriovenous difference that was never significantly different from zero. To establish a basis for this lack of clearance, we investigated uptake and hydrolysis of glycylglutamine at the blood-brain barrier. The study of brain uptake index of glycylglutamine in rats showed that it was not significantly different from that of sucrose, an impermeable marker (3.2 +/- 0.4 v 3.5 +/- 0.4, n = 4 to 5). When isolated brain capillaries were incubated with glycylglutamine, uptake was only modestly above background activity and appeared to be due to nonspecific binding. Finally, the plasma membrane of brain capillaries lacked hydrolase activity against glycylglutamine. In conclusion, brain appears to be unique among organs in lacking any mechanism for clearance of glycylglutamine from plasma.