Kinetic analysis of L-leucine transport across the blood-brain barrier.

Unidirectional L-leucine influx across cerebral capillaries was measured at different concentrations with an in situ rat brain perfusion technique, which has several advantages over presently-used methods such as the Brain Uptake Index (BUI) technique. The maximal influx rate (Vmax) and half-saturation concentration (Km) equaled 1.07 +/- 0.02 X 10(-3) mumol X s-1 X g-1 and 0.026 +/- 0.002 mumol X ml-1, respectively, for the saturable component, and the constant for non-saturable influx equaled 6.8 +/- 1.4 X 10(-5) s-1. These values differ by 3-4-fold from respective values obtained with the BUI technique.