The endothelia of microvessels isolated from mouse brain by mechanical means are rich in gamma-glutamyl transpeptidase; however, the enzyme often disappears when the cells migrate or proliferate from the microvessel isolates. In an endothelial cell line derived from similar isolates and negative for gamma-glutamyl transpeptidase, the enzyme could be induced in the endothelial cells when they were cocultured with glial cells. Thus there may be a requirement for continuous induction of gamma-glutamyl transpeptidase in brain microvessels by adjacent glial cells.

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