Effects of thiol-protecting reagents on the size of solubilized adenylate cyclase and on its ability to be stimulated by guanyl nucleotides and fluoride

Properties of membranous phospholipase C from WRK1 cell: sensitivity to guanylnucleotides and bacterial toxins

As previously described, WRK1 plasma membrane possesses a vasopressin-sensitive phospholipase C [G. Guillon et al., 1986, FEBS Lett. 196, 155-159]. In the present study, we examined the sensitivity of this enzyme to guanylnucleotides. GTP gamma S induces a time- and dose-dependent stimulation of Ins(1,4,5)P3 and Ins(1,4)P2 accumulation. No accumulation of InsP1, Ins(1,3,4)P3 or Ins(1,3,4,5)P4 occurred under similar conditions. Gpp(NH)p produced the same effect but was less potent. GTP and a nonhydrolyzable analogue of ATP, App(NH)p, were without effect. Calcium also stimulated the phospholipase C activity in a time- and dose-dependent manner. In the absence of calcium, the activity of GTP gamma S was considerably reduced. Physiological calcium concentrations (between 10(-8) and 10(-7) M), allowed maximal GTP gamma S stimulation of phospholipase C activity. In this system, the presence of vasopressin alone did not generate inositol phosphate accumulation. However, this hormone: (i) reduced the lag-time observed during GTP gamma S stimulation, (ii) increased the sensitivity of phospholipase C to GTP and to GTP gamma S, and (iii) did not modify the stimulation of phospholipase C induced by maximal doses of GTP gamma S. Unlike sodium fluoride, GTP gamma S elicited an irreversible activation of phospholipase C. Calcium, GTP gamma S and sodium fluoride stimulated the phospholipase C activity via mechanisms sharing a common step, since their maximal effects were not additive. Cholera toxin treatment, known to produce complete ADP-ribosylation of 'alpha s' subunits, partially reduced the basal and the maximal GTP gamma S-mediated stimulation of phospholipase C activity as well as that caused by vasopressin. This inhibition was not mimicked by treatment with either forskolin or pertussis toxin.

Inhibition of adenylate cyclase activity during the reversible shape change in human platelets

Washed platelets in the absence of physiological activators possess gravity inducible shape change, which is paralleled by an increase of phosphatidic acid (PA), polyphosphoinositides (PPI) and an inhibition of PGE1 and Gpp(NH)p- stimulated adenylate cyclase (AC) activity. Incubation of platelets at 37 degrees C for 1 hr decreases (32P)PPI and restores their response to low doses of thrombin (0.015 U/ml). Simultaneously an increase of PGE1- and Gpp(NH)p- stimulation of AC is observed. The relaxation of the platelets influences predominantly the cAMP levels without significantly affecting the dissociation constants of the stimulators. Forskolin-induced activation of AC is the same in stimulated and relaxed platelets. It is suggested that the initial increase of PA inhibits the coupling of regulatory and receptor proteins to AC and has no effect on the catalytic unit.