Influences of silicates and carnitine-silicate mixtures on the inhibition of aggregation of erythrocytes elicited by the presence of fibrinogen.

Carnitine and acylcarnitine derivatives have been reported to inhibit cell aggregation (Fritz and Burdzy, 1989, J. Cell. Physiol., 140:18-28). A follow-up of these observations showed that whereas the previously described effects of long-chain acylcarnitines were well replicated, those of carnitine on erythrocytes showed marked variability. The latter phenomenon was traced to the presence of silicates in carnitine solutions derived from the use of sodium hydroxide solutions stored in glass containers for the neutralization of carnitine. The present experiments have led to the discovery that oligomeric forms of silicates are powerful inhibitors of red blood cell aggregation which otherwise occurs in the presence of fibrinogen alone. The active form(s) of silicates in this assay, which appear to be generated by polymerization of silicates in metasilicate solutions on neutralization, are unstable and therefore transient under usual conditions. We estimate that the active oligomeric forms contain between 4 to 18 silicon atoms per molecule. When maintained at -18 degrees C in the presence of carnitine, but not in its absence, the active forms of oligomeric silicates remained stable for months, judging from their ability to inhibit cell aggregation. We conclude that carnitine stabilized the oligomeric form(s) of silicate, or that the species stabilized is an oligomeric silicate-carnitine complex. Comparable concentrations of choline, deoxycarnitine, or gamma-aminobutyrate were less effective in stabilizing the active silicate oligomers. The active forms of the silicate oligomers had Ki values of about 10 microM, calculated as the monomeric form, in inhibiting red blood cell aggregation. The data indicate that free carnitine does not directly inhibit erythrocyte inhibition, as previously interpreted, whereas long-chain acylcarnitine derivatives are active in the absence of silicates. Possible mechanism of actions of silicate oligomers on membranes are discussed.