TABLE 2.

Comparison of large neutral amino acid Km values in isolated human brain capillary in vitro and rat blood-brain barrier in vivo.

Neutral amino acid	Km (uM) Human brain microvessel	Km (uM) Rat BBB in vivo
Phenylalanine	0.30 ± 0.08	6.9 ± 0.6
Tyrosine	1.3 ± 0.4	16.1 ± 1.1
Isoleucine	2.7 ± 1.1	10.0 ± 0.7
Tryptophan	3.0 ± 0.7	9.1 ± 0.8
Leucine	3.3 ± 0.7	9.9 ± 0.9
Methionine	5.1 ± 2.5	4.2 ± 1.0
Histidine	5.1 ± 2.0	10.2 ± 0.4
Valine	8.8 ± 4.6	8.2 ± 0.5

Human brain microvessel Km values in vitro from Hargreaves and Pardridge (1988); rat BBB Km values in vivo from Smith et al. (1987).