With dimethyl sulphoxide instead of butanol in the thiobarbituric acid assay for sialic acid, a non-fading chromophore with lambdamax. = 549 nm was produced in a homogeneous solution, allowing dilution of the test mixture in case of high colour yield. This test adapted well to studies on alkaline de-O-acetylation. Bovine and rat submaxillary mucins, and rabbit Tamm-Horsfall urinary sialoproteins contain O-acetyl isomers of neuramine acid that are resistant to the thiobarbituric acid assay. Alkaline de-O-acetylation converted resistant O-acetylneuraminic acid into thiobarbituric acid-reactive sialic acid, and such conversion paralleled de-O-acetylation as measured by the ferric hydroxamate method. The colour increment was similar when the alkaline treatment of bovine submaxillary mucin either preceded or followed the acid hydrolysis. Only alkaline preptreatment was effective with rat submaxillary mucin. By selecting optimal conditions for alkaline de-O-acetylation, O-acetyl isomers can be accurately assessed by the thiobarbituric acid assay.

The high content of sialic acid in the glomerulus is associated with the cell membrane of epithelial cells lining the basement membrane. Whereas enzyme studies indicate that sialic acid is a determinant of the nephritogenic antigen, the physicochemical properties of this nephritogenic glycoprotein suggest that sialic acid may have an important role in the filtration mechanism.

SummaryThe kinetics, activation, pH dependence and inhibition of platelet aggregation induced by ADP were studied by recording the OD of stirred PRP from blood anticoagulated with acid-citrate-dextrose solution. There was a lag period between the addition of ADP and the initiation of aggregation. Addition of ADP in the absence of stirring or ionized calcium did not cause aggregation. When aggregation was subsequently initiated by stirring or the addition of CaCl2 there was no lag period. The steepest slope of the OD curve was used as the reaction velocity (V). It was affected by the concentration of calcium ions and was maximum at pH 8.0. When the ADP concentration was varied in the presence of different concentrations of ionized calcium, the overall kinetics revealed a series of rectangular hyperbola segments which did not pass through the origin. These observations led to the conclusion that the overall reaction can be depicted as a chain reaction: ADP interacts reversibly with platelets; when calcium is present these platelets become “sticky” and, when stirred, they aggregate. Kinetic studies of inhibition indicated that adenosine is a competitive inhibitor of ADP. The guanidino compounds tested were noncompetitive with respect to ADP but their inhibitory activity was counteracted by calcium.

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