Inhibition of N-methyl-N'-nitro-N-nitrosoguanidine-activated guanylate cyclase by anticarcinogenic agents

Recent studies have demonstrated that nitroso chemical carcinogens markedly activate guanylate cyclase, which catalyzes the production of guanosine 3',5'-monophosphate (cyclic GMP). We therefore examined the effect of inhibitors of carcinogenic compounds on guanylate cyclase activation by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). An antioxidant group of anticarcinogenic compounds was effective. Disulfiram and phenethyl isothiocyanate exhibited the most potent inhibition. Inhibitor constants (Ki) for disulfiram and phenethyl isothiocyanate were 1.2 x 10(-5) M and 4.9 x 10(-5) M, respectively. Sodium diethyldithiocarbamate, phenyl isothiocyanate, butylated hydroxyanisole and ethoxyquin showed moderate inhibitory effects. Sodium selenide decreased the MNNG-activated guanylate cyclase activity to about 30%, and it was inhibitory at the low concentration of 10(-5) M. The present data suggest that one of the mechanisms by which anticarcinogenic compounds exert their effect may in part be related to the inhibition of guanylate cyclase.

Subsynaptosomal distribution of 45Ca taken up by synaptosomes in high-potassium medium

The 45Ca uptake of synaptosomes was stimulated by high K+, and the K-stimulated uptake was temperature dependent and reached a plateau level within 20 sec at 30 degrees C. The synaptosomes which took up 45Ca in high-K+ medium for 20 sec was disrupted, and subsynaptosomal distribution of 45Ca was examined. The percentage increases of 45Ca labeling of fractions D (vesicles), C (myelin), B (synaptic plasma membrane), and A (mitochondria) induced by high potassium were 27.7 +/- 10.3%, 28.7 +/- 10.4%, 31.0 +/- 4.0%, and 48.1 +/- 5.5%, respectively. However, total counts of K-stimulated 45Ca labeling in fraction B was equal to that in fraction A, and those in fractions C and D were less. These observations indicate that Ca binding on the synaptic plasma membrane may be as important as a Ca reservoir or regular of Ca2+ in nerve terminals as the mitochondria.

Effect of L-leucine on p-aminohippurate accumulation by mammalian renal cortical slices

The effect of several amino acids (L-glutamate, L-phenylalanine, L-leucine, glycine, L-tryptophan, L-histidine, L-valine) on p-aminohippurate accumulation was evaluated in rat, mouse and rabbit kidneys. Only leucine was found to enhance p-aminohippurate accumulation in rat and mouse renal cortical slices but not in rabbit slices. Leucine had no effect on tetraethylammonium accumulation. In rat renal cortical slices, leucine increased the uptake and decreased the runout of p-aminohippurate, each effect contributing to the increase in p-aminohippurate accumulation. The apparent Km of p-aminohippurate uptake was decreased by leucine with no change in the apparent V. Aminooxyacetate (an inhibitor of transamination of leucine) partially depressed the stimulating effect of leucine on p-aminohippurate accumulation, whereas alpha-ketoisocaproic acid (a metabolite of leucine formed by transamination) enhanced p-aminohippurate accumulation, suggesting that the metabolism of leucine in kidney slices may be necessary for the stimulating effect on p-aminohippurate transport.

Subcellular ascorbic acid in scorbutic guinea pig brain

In scorbutic guinea pigs fed an ascorbic acid-deficient diet for 18 days, the ascorbic acid contents of the liver and plasma were less than 4% those of the control animals, while the content in the brain was about one-third that of the controls. In normal animals, the concentration of ascrobic acid was highest (7.55 +/- 1.46 micrograms of ascorbic acid/mg protein) in the S3 fraction and was also fairly high (5.37 +/- 0.91) in the P2p fraction (cytoplasm of nerve terminals). During ascorbic acid deficiency, the contents in the P2p and microsome fractions decreased slightly faster than those in other fractions. Even after 18 days' deficiency, no significant change was found in the acetylcholine content of the brain.

Effects of synaptic plasma membranes on release of acetylcholine from synaptic vesicles

The influences of synaptic plasma membranes on release of acetylcholine (ACh) from synaptic vesicles isolated from rat brain were examined. In the presence of ATP, Mg++ and Ca++ but absence of cytoplasm from the nerve endings, the synaptic plasma membranes did not increase ACh release indicating absence of a stimulating factor which is known to be present in the cytoplasm. In presence of ATP, Mg++, Ca++ and the cytoplasm, the synaptic plasma membranes inhibited ACh release from the synaptic vesicles in high K+ medium, though not in high Na+ medium. Binding of Ca++ by the synaptic plasma membranes was dependent on ATP, inhibited by Na+ and stimulated by K+. Thus, the synaptic plasma membranes may inhibit ACh release in high K+ medium due to reduction in the concentration of free Ca++.

Bound forms of Ca taken up by the synaptic plasma membrane

Temperature dependent Ca-binding by the synaptic plasma membrane was increased in the presence of ATP and Mg++. Apparent Km for ATP was about 2.8 X 10(-5) M and optimal concentration of Mg++ was 2 mM in the presence of 2 mM ATP. After preincubation with nonradioactive Ca++, ATP and Mg++ to attain a steady state, addition of 45Ca resulted in remarkable labelling of the membrane, indicating rapid turnover of most of the membrane bound Ca. The presence of oxalate (60 mM) greatly increased Ca up-take on prolonged incubation. The Ca uptake in presence and absence of oxalate had similar substrate specificity and was similarly influenced by various monovalent cations. Furthermore, activities for Ca-uptake in the presence and absence of oxalate could not be separated by sucrose density gradient centrifugation of the synaptic plasma membrane fraction. Accordingly, it was considered that Ca++ in the medium was taken up by surface of the membrane, ATP- and temperature-dependently and then transferred into a cavity where the Ca-oxalate complex is formed.

Effects of La+++, Mn++ and ruthenium red on Mg-Ca-ATPase activity and ATP-dependent Ca-binding of the synaptic plasma membrane

The effects of La+++, Mn++ and ruthenium red (R.R.) on Ca-uptake of synaptic plasma membranes (S.P.M.) were investigated. La+++ (0.1 mM), Mn++ (0.2 mM) and R.R. (0.1 mM) selectively inhibited Mg-Ca-ATPase but did not significantly affect Mg-ATPase activity. The apparent Ki values of La+++, Mn++ and R.R. for Mg-Ca-ATPase were 0.05, 0.06 and 0.03 mM, respectively. La+++, Mn++ and R.R. did not affect Ca-uptake at concentrations which strongly inhibited Mg-Ca-ATPase activity. These results indicate that Ca-uptake by S.P.M. differ from that by sarcoplasmic reticulum.