Periodate-induced increase in cyclic GMP in mouse and guinea pig splenic cells in association with mitogenesis

The involvement of catalytic site thiol groups in the activation of soluble guanylate cyclase by sodium nitroprusside

Sodium nitroprusside, a potent activator of soluble guanylate cyclase, potentiated mixed disulfide formation between cystine, a potent inhibitor of the cyclase, and enzyme purified from rat lung. Incubation of soluble guanylate cyclase with nitroprusside and [35S]cystine resulted in a twofold increase in protein-bound radioactivity compared to incubations in the absence of nitroprusside. Purified enzyme preincubated with nitroprusside and then gel filtered (activated enzyme) was activated 10- to 20-fold compared to guanylate cyclase preincubated in the absence of nitroprusside and similarly processed (nonactivated enzyme). This activation was completely reversed by subsequent incubation at 37 degrees C (activation-reversed enzyme). Incorporation of [35S]cystine into guanylate cyclase was increased twofold with activated enzyme, while no difference was observed with activation-reversed enzyme, compared to nonactivated enzyme. Cystine decreased the activity of nonactivated and activation-reversed enzyme about 40% while it completely inhibited activated guanylate cyclase. Mg+2- or Mn+2-GTP inhibited the incorporation of [35S]cystine into nonactivated or activated guanylate cyclase. Also, diamide, a potent thiol oxidant that converts juxtaposed sulfhydryls to disulfides, completely blocked incorporation of [35S]cystine into nonactivated or activated guanylate cyclase. These data indicate that activation of soluble guanylate cyclase by nitroprusside results in an increased availability of protein sulfhydryl groups for mixed disulfide formation with cystine. Protection against mixed disulfide formation with diamide or substrate suggests that these groups exist as two or more juxtaposed sulfhydryl groups at the active site or a site on the enzyme that regulates catalytic activity. Differential inhibition by mixed disulfide formation of nonactivated and activated enzyme suggests a mechanism for amplification of the on-off signal for soluble guanylate cyclase within cells.

Effects of thiol inhibitors on hepatic guanylate cylase activity

Several thiol blocking agents inhibit basal guanylate cyclase activity of 100 000 X g hepatic supernatant fractions and the stimulation of enzyme activity by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), NaN3, NaNO2 and nitroprusside. The relative potency of the thiol blockers as inhibitors was CdCl2 greater than p-hydroxymercuribenzoate greater than N-ethylmaleimide greater than arsenite greater than iodoacetamide. Inhibition of basal and MNNG-responsive soluble guanylate cyclase activities by arsenite was markedly potentiated by an equimolar concentration of 2,3-dimercaprol, but not by mercaptoethanol. Inhibition of soluble guanylate cyclase by either arsenite or CdCl2 was completely reversed by excess 2,3-dimercaprol. Qualitatively similar effects were observed with DE-52 cellulose purified soluble hepatic guanylate cyclase, and suggested an involvement of closely juxtaposed thiol groups in the regulation of enzyme activity. For several reasons inhibition by thiol blockers appeared to be mediated through multiple mechanisms and/or sites of interaction: (1) Concentrations of the thiol inhibitors which had no effect on basal activity strikingly inhibited the responsiveness of the enzyme to a submaximal concentration of MNNG. (2) CdCl2 abolished the action of excess MnCl2 to stimulate purified guanylate cyclase, but was a relatively ineffective inhibitor when MnCl2 and GTP were present in equimolar concentrations. By contrast, arsenite-2,3-dimercaprol was uniformly effective in inhibiting guanylate cyclase activity in the presence or absence of excess MnCl2. (3) Arsenite-2,3-dimercaprol increased the Km for MnGTP (control, 0.13 +/- 0.02 mM; 0.2 mM arsenite-2,3-dimercaprol, 0.31 +/- 0.03 mM), whereas CdCl2 had no effect on this parameter. (4) Hepatic particulate guanylate cyclase activity was significantly inhibited by arsenite 2,3-dimercaprol but not by CdCl2. Thus, the data not only indicate that vicinal dithiol groups are required for expression of basal guanylate cyclase activity and enzyme responses to agonists, but strongly suggest the involvement of more than one interacting site containing free thiol residues.

A simplified procedure for cyclic nucleotide radioimmunoassay and its application to human blood leukocytes

Rapid treatment of leukocyte suspensions was found to be an effective alternative to acid treatment in the preparation of these cells for radioimmunoassay of cyclic GMP and cyclic AMP. A 2-sec heating to boiling temperature, followed by sonication and micropore filtration, was employed. This procedure adequately inactivated or removed enzymes and binding proteins that can alter cyclic nucleotide concentrations or otherwise interfere with the radioimmunoassay. Moreover, heating in this manner did not appear to affect the stability of cyclic nucleotides or cause significant formation of cyclic GMP from endogenous GTP. Recovery of cyclic nucleotides after heating and filtration was high (89-96%), making possible the measurement of both cyclic GMP and cyclic AMP in small cellular samples. Variation in cyclic nucleotide recovery was small (+/- 2-4% SD), therefore individual recovery determinations were unnecessary.

Activation of the guanylate cyclase-guanosine 3'5' monophosphate system of colonic mucosa by n-methyl-n'-nitro-n-nitrosoguanidine

The effects of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) on the guanylate cyclase (GC)-guanosine 3'5' monophosphate (cGMP) system of rat colonic mucosa were studied. MNNG (1 mM) increased colonic mucosal cGMP from 1.8 +/- 0.2 to 22.5 +/- 2.7 pmol/mg protein in 5 minutes. Increases in response to MNNG occurred in the presence or absence of extracellular Ca2+, whereas the two-fold increase in mucosal cGMP mediated by carbamylcholine was abolished by exclusion of Ca2+. Although GC activity of mucosal homogenates was found predominantly (90%) in the 100,000 g particulate fraction, the effects of MNNG on mucosal cGMP correlated with stimulation of 100,000 g soluble GC by this agonist. MNNG increased soluble GC 13-fold over the corresponding basal with 4 mM Mn2+, and 48-fold with 4 mM Mg2+ as the sole available divalent cation. Compared with unstimulated GC, the MNNG-activated soluble enzyme was less dependent upon Mn2+ availability and effectively utilized Mg2+ as metal co-factor. N-ethylmaleimide, a sulfhydryl group alkylator, inhibited MNNG stimulation of GC and cGMP. Thus, expression of these MNNG actions may involve drug interaction with tissue thiol groups. Prior incubation of MNNG with thiol antioxidants or ascorbate also suppressed MNNG stimulation of GC, possibly through direct drug reactions involving nucleophilic and electrophilic reactants. The ability of MNNG to stimulate the colonic mucosal GC-cGMP system could be linked to its carcinogenic action.

Cell surface changes accompanying myoblast differentiation

Myoblasts are mononucleated cells and associated with differentiation undergo cell fusion and become multinucleated. The current studies have examined cell surface dynamic changes of Concanavalin A lectin receptor mobility and the role of hormones in modulating myoblast differentiation. A uniform distribution of Con-A receptors is observed in undifferentiated cells when reacted with Con-A at 37 degrees C. Cells from differentiating cultures or fully differentiated myotubes reacted similarly at 37 degrees C show a significant redistribution of Con-A into patches, "caps," and endocytic vesicles containing Con-A. If undifferentiated and differentiated cells are first prefixed with glutaraldehyde then reacted with Con-A continuous distribution of Con-A is seen across the cell surface. This suggests redistribution of Con-A and its receptors occurs in differentiated cells reacted with lectin at 37 degrees C. It is further shown that insulin (10 microgram/ml) significantly enhances myoblast differentiation but that this occurs after an apparent stimulation of proliferation. In contrast to insulin, dexamethasone (10 micron and 100 micron) profoundly inhibits myoblast differentiation while having different effects on proliferation; 10 micron dex stimulates cell growth while 100 micron dex suppresses cell proliferation. Lastly, an extracellular filamentous matrix which binds Con-A is observed at the ultrastructural level in high density cultures. No significant redistribution of Con-A is observed on this matrix in distinction to the redistribution observed on the cell membrane in differentiated cells.