Four antibiotics which belong to the vancomycin group have been examined in respect of their effects on the factor VIII dependent platelet agglutination. A striking similarity between ristocetin and ristomycin was observed both in qualitative and quantitative terms, therefore ristomycin could be used to determine the so called ristocetin cofactor. Actinoidin and vancomycin inhibited platelet agglutination induced by ristocetin or ristomycin in citrate-PRP or EDTA-PRP as well as in systems containing formaldehyde-treated platelets, but did not inhibit agglutination induced by bovine factor VIII. All the four antibiotics caused plasma protein precipitation. Actinoidin was the least and vancomycin the most effective in this respect; ristocetin and ristomycin also possess this property, the effect of the latter is more considerable. Actinoidin and vancomycin did not block the immediate increase in light absorbancy in aggregometer caused by the addition of ristocetin or ristomycin to fixed platelets at concentrations which totally inhibited platelet agglutination in the presence of protein cofactor. Inhibition of this 'direct effect' of ristocetin and ristomycin was observed only at higher concentrations indicating that this effect may be unrelated to the agglutination. According to our results with ristomycin derivatives the methylated carboxyl and the free phenolic hydroxyl groups may be of prime importance in the binding of ristomycin to the platelet membrane and/or to its protein cofactor. Similar data from other laboratories are confirmed and some new findings are offered.
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