Activity of membrane-bound gamma-glutamyl transpeptidase (gamma-GTP) was examined in various regions of mouse brain, in capillaries of the cerebral cortex and in telencephalic choroid plexuses. The level of activity in the capillaries was double and that of the choroid plexus nine times that of the gamma-GTP activity found in the brain, septum, hippocampus, hypothalamus, thalamus, cerebellum, frontal cortex, pons, medulla oblongata, and amygdala. Histochemically the gamma-GTP activity was demonstrated in the surface membranes of choroidal cells and in the endothelium of small capillaries. The activities of gamma-GTP of cerebral cortex, choroid plexus, and capillaries from rabbit were 5--17 times greater than those from corresponding areas of mouse brain. While 30 mM methionine stimulated (in vitro) the enzyme from mouse brain, no such effect was observed with the enzyme activity from rabbit brain. The gamma-GTP activity from the capillaries of cerebral cortex of both mouse and rabbit was not affected by the presence of methionine. These findings suggest existence of differences in the specificity of gamma-GTP activity in these two species.

Many extracellular signals are at the cell surface received by specific receptors, which upon activation transduce information to the appropriate cellular effector molecules via trimeric G proteins. The G protein-mediated cascades ultimately lead to the highly refined regulation of systems such as sensory perception, cell growth, and hormonal regulation. Transmembrane signaling may be seriously deranged in various pathophysiological conditions. Over the last two decades the major experimental effort of our group has been devoted to better understanding the molecular mechanisms underlying transmembrane signaling regulated by G proteins and to the closely related process of desensitization of hormone response. This review provides general information about the basic principles of G protein-regulated transmembrane signaling as well as about our contribution to the current progress in the field.

A relationship between membrane-bound gamma-glutamyl transpeptidase (GGT) in liver and choroid plexus and the soluble GGT in blood plasma and cerebrospinal fluid is indicated by a decrease of the former activity in the organs and a concomitant increase of the latter in the body fluids of the embryonic chick between day 11 and hatching. Starting on day 15, the administration of cortisol caused a marked increase in the activity of soluble GGT in blood plasma and cerebrospinal fluid which might be related to an increase of the membrane-bound activity of this enzyme by the glucocorticoid in liver, brain and choroid plexus during the same developmental period. However, before day 15 cortisol reduced GGT activity in the liver and choroid plexus suggesting a biphasic effect of the steroid on the activity of membrane-bound GGT.