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The effects and modes of action of certain lipid second messengers and protein kinase C regulators, such as sphingosine, lysophosphatidylcholine (lyso-PC), and oleic acid, on Na,K-ATPase and sodium pump were examined. Inhibition of purified rat brain synaptosome Na,K-ATPase by these lipid metabolites, unlike that by ouabain, was subject to membrane dilution (i.e. inhibition being counteracted by increasing amounts of membrane lipids). Kinetic analysis, using the purified enzyme, indicated that sphingosine and lyso-PC were likely to interact, directly or indirectly, with Na+-binding sites of Na,K-ATPase located at the intracellular face of plasma membranes, a conclusion also supported by studies on Na,K-ATPase and 22Na uptake using the inside-out vesicles of human erythrocyte membranes. The studies also showed that ouabain (but not sphingosine and lyso-PC) increased the affinity constant (K0.5) for K+, whereas sphingosine and lyso-PC (but not ouabain) increased K0.5 for Na+. Sphingosine and lyso-PC inhibited 86Rb uptake by intact human leukemia HL-60 cells at potencies comparable to those for inhibitions of purified Na,K-ATPase and protein kinase C. It is suggested that Na,K-ATPase (sodium pump) might represent an additional target system, besides protein kinase C, for sphingosine and possibly other lipid second messengers.
Oishi et al. (Mon,) studied this question.