A simple method for estimating a 'heparin binding capacity' of human serum

A method for estimating a 'heparin binding capacity' of human serum is described. When human serum is diluted with a low ionic strength, heparin-containing buffer at pH 5.5, protein-heparin electrostatic complexes form in solution with subsequent formation of insoluble aggregates which can be collected by centrifugation. Quantitative determination of the relative amounts by weight of protein and heparin in the insoluble heparin-protein aggregates permits estimation of a combining ratio at which serum proteins bind with heparin to precipitate from solution. This weight combining ratio of protein and heparin is a quantitative measure of the total affinity for heparin of all proteins in serum which bind heparin at pH 5.57 to form an insoluble complex. An unusually high affinity for heparin by an abnormal serum protein or an increase in amount of a normally-occuring, high heparin-affinity, serum protein would alter the average protein: heparin combining ratio and increase the 'heparin-binding capacity' of human serum. The converse would be true for serum proteins having a low affinity for heparin, lowering the 'heparin-binding capacity' of human serum. The described method was used to evaluate the 'heparin-binding capacity' of serum proteins in normal individuals and in persons with cystic fibrosis.