Binding of noradrenaline to bovine serum albumin

Abstract

The binding of noradrenaline to bovine serum albumin (40 mg ml −1 ) has been studied by gel filtration. Over the range of 5 × 10 −9 M to 10 −3 M of noradrenaline, the data obtained fit a model with three classes of noninteracting binding sites: a high affinity-low capacity site with a K A of 1.18 × 10 6 M −1 and a capacity of 11.84 pmol per mg of protein; an intermediate site with a K A of 7.42 × 10 4 M −1 and a capacity of 57.25 pmol mg −1 ; and a low affinity-high capacity site with a K A of 8.64 × 10 2 M −1 and a capacity of 1265 pmol mg −1 . There are no significant differences in binding among the three bovine serum albumin preparations considered. Both (±) and (−) noradrenaline are bound equally well by albumin. The binding is a relatively slow process but has a very rapid initial rate, about 40-fold higher than the observed at the half-maximal binding. At pH 7.4 the binding capacity is about 1.7-times higher and the initial rate is about 10-fold higher than at pH 5, but the affinity is similar. There is no degradation of noradrenaline in the presence of serum albumin. The reversibility of binding is small by dialysis or acidic precipitation. At physiological concentrations of noradrenaline and serum albumin it is estimated that about 60% of noradrenaline would be bound. The possible physiological significance of binding is discussed.