Inhibition of Mg- and Mg, Na, K-activated adenosine tgriphosphatase activity by L-ascorbic acid and L-cysteine

A comparative study between a brain Na+,K(+)-ATPase inhibitor (endobain E) and ascorbic acid

In the search of Na+,K(+)-ATPase modulators, we have reported the isolation by gel filtration and HPLC of a brain fraction, termed endobain E, which highly inhibits Na+,K(+)-ATPase activity. In the present study we compared some properties of endobain E with those of ascorbic acid. Kinetic experiments assaying synaptosomal membrane K(+)-p-nitrophenylphosphatase (K(+)-p-NPPase) activity in the presence of endobain E or ascorbic acid showed that in neither case did enzyme inhibition prove competitive in nature versus K+ or p-NPP concentration. At pH 5.0, endobain E and ascorbic acid maximal UV absorbance was 266 and 258 nm, respectively; alkalinization to pH 14.0 led to absorption drop and shift for endobain E but to absorbance disappearance for ascorbic acid. After cysteine treatment, endobain E absorbance decreased, whereas that of ascorbic acid remained unaltered; iodine treatment led to absorbance drop and shift for endobain E but to absorbance disappearance for ascorbic acid. HPLC analysis of endobain E disclosed the presence of two components: one eluting with retention time and UV spectrum indistinguishable from those of ascorbic acid and a second, as yet unidentified, both exerting Na+,K(+)-ATPase inhibition.

Ascorbic acid: an endogenous inhibitor of isolated Na+,K+-ATPase

During attempts to isolate and identify an endogenous ligand for the glycoside binding sites on Na+,K+-ATPase, bovine adrenal glands were found to contain a potent inhibitor of isolated Na+,K+-ATPase. The inhibitory principle was extracted from adrenal cortex, following homogenization in NaHCO3 solution and separation on a Sephadex G-10 column. The active principle was recovered from a fraction which eluted from the column after the 3H2O peak. The extract inhibited isolated Na+,K+-ATPase and the specific [3H]ouabain binding reaction. Sensitivity of the enzyme to the inhibitory action of the extract was species and tissue dependent; however, the pattern and the magnitude of the sensitivity were different from those of the digitalis glycosides. Moreover, the inhibitory principle failed to inhibit sodium pump activity, estimated from ouabain inhibitable 86Rb+ uptake by guinea pig brain slices. The activity of the extract to inhibit isolated Na+,K+-ATPase was stable under acidic condition but was lost rapidly at neutral pH, and could be eliminated by EDTA. In an acidic medium, the inhibitory principle had an absorption maximum at 244 nm which shifted to 264 nm and decayed rapidly at neutral pH. By using mass spectrometry, the principle was identified to be ascorbic acid, which has been shown previously to inhibit isolated Na+,K+-ATPase under appropriate conditions. Because ascorbic acid was incapable of inhibiting the sodium pump in intact cells, this inhibitor of the isolated enzyme does not appear to be the endogenous ligand which regulates sodium pump activity in vivo.

Vanadyl (VO2+) induced lipoperoxidation in the brain microsomal fraction is not related to VO2+ inhibition of Na,K-ATPase

Vanadyl (VO2+) is a potent inductor of the lipid peroxidation in brain microsomes. This effect, however, is obtained at concentrations by two orders of magnitude higher (10(-4)-10(-3)M) than those which effectively inhibit the brain microsomal Na,K-ATPase. At 10(-6)M VO2+ which inhibits 50% of the Na,K-ATPase activity there is no measurable malonyldialdehyde production. Vanadate (VO-3) which is an equally potent inhibitor of Na,K-ATPase as VO2+ has almost no capacity to induce the lipoperoxidation. The addition of 10(-4)M ascorbate to the brain microsomes stimulates the lipoperoxidation to the maximum level regardless of the presence or absence of exogenous vanadium ions. Ascorbate-induced inhibition of brain Na,K-ATPase which is known to be associated with lipoperoxidation is strictly additive with the vanadyl (VO2+) inhibition of this enzyme. Even at submaximal concentrations there is no indication for any potentiation between these two inhibitory systems. The disparity between the mechanisms of ascorbate and vanadyl-induced inhibition of Na,K-ATPase is also documented by the effect of EDTA which inhibits the former type only. It is concluded, that the vanadium-induced inhibition of brain microsomal Na,K-ATPase is not related to induction of lipoperoxidative capacity of the brain.

In vitro and in vivo depolarization coupled efflux of ascorbic acid in rat brain preparations

The depolarization-coupled efflux of endogenous ascorbate is demonstrated using rat synaptosome preparations and a rat cortical cup method. Ascorbate and catecholamines (monitored as a methodological control) were quantitated by high pressure liquid chromatography with electrochemical detection. In vitro potassium ion induces ascorbate efflux in a concentration-dependent, although calcium ion-dependent, manner. Veratridine also induces ascorbate efflux and its effect can be blocked by tetrodotoxin. In vivo, ascorbate efflux was likewise stimulated by increased potassium ion and by veratridine. In addition, electrical stimulation of medial lemniscus was accompanied by an increased efflux of ascorbate from somatosensory cortex. These results are intriguing in light of the recent evidence for the interaction of ascorbate with several neurotransmitter systems.

Ascorbic acid in fetal human brain

Ascorbic acid concentrations in fetal human forebrain in the period 11 to 19 weeks' gestational age were 4 to 11 times higher than those of adults. Levels fell progressively with increasing gestational age but, in term babies dying within 4 weeks of birth, were still at least 3 times those of adults. It was confirmed that, in women delivering at term, ascorbic acid concentrations are approximately 4 times higher in cord blood plasma than in maternal blood plasma. The possible importance of ascorbic acid for normal human brain development is discussed.