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The interaction of human serum albumin and of alpha-fetoprotein to poly(ethylene glycol)-bound fatty acids is compared using the method of affinity partitioning in an aqueous two-phase system composed of dextran, poly(ethylene glycol) and esters of different fatty acids with poly(ethylene glycol). Albumin was found to be bound to the fatty acid derivatives with higher affinity than alpha-fetoprotein. For both proteins binding occurred in dependence on the chain length of the fatty acids with a maximum for the C14-derivative.

The partition of serum albumin between the two aqueous phases of a polyethylene glycol-dextran-water two-phase system is strongly influenced by replacing a small fraction of the polyethylene glycol by its palmitate ester. The partition of albumin is compared with the partition of polymer-bound palmitate using polyethylene glycol [1-14C]palmitate. The data indicate that a maximum ("saturation") effect on the partition of albumin corresponds to a binding of fewer than two palmitate groups per albumin molecule. The effect on alpha-lactalbumin corresponds to the maximum binding of 0.5 palmitate group per protein molecule. These data do not fit the most favoured available model for affinity partitioning. The deviation can be attributed to the observation that the palmitate groups probably form micelle-type aggregates in the polyethylene glycol-rich phase at the concentrations used for albumin extraction.

The interaction of human serum albumin with fatty acids has been determined using the method of affinity partitioning in aqueous biphasic systems containing dextran, poly(ethylene glycol) and esters of dicarboxylic acids with poly(ethylene glycol). The difference in the partition of albumin in phase systems with and without the poly(ethylene glycol)-bound fatty acid group provides a measure of the interaction of fatty acids with the protein. The relative contribution of the polar and non-polar interaction to the binding of fatty acids to albumin has been estimated by comparing the present data with that obtained earlier using poly(ethylene glycol)-bound straight chain aliphatic hydrocarbons. In both cases, the aliphatic chain should contain a minimum of 8 carbon atoms to affect the partition of albumin and that the maximum effect is obtained with chains containing 16 carbon atoms. The effect of the polymer-bound fatty acid group is higher than the corresponding hydrocarbon only when the number of carbon atoms in it exceeds 12. The relative effect of polymer-bound 16-carbon chains, with and without a carboxyl group in the terminal position is independent of pH in the range 5--9.

A mathematical model for the calculation of free and protein bound concentrations of testosterone and estradiol in plasma is presented. The method is based on the knowledge of the total concentrations of all steroids competing for the same binding site on testosterone-estradiol-binding globulin (TeBG), the concentration of albumin, the binding capacity of TeBG, and the association constants of the steroids to the two binding proteins. For the calculations we have determined the total concentrations of testosterone and estradiol. TeBG binding capacity, albumin concentration and the association constants for the binding of testosterone, estradiol and 5 alpha-dihydrotestosterone (DHT) to TeBG and albumin at 37 degrees C. Physiological concentrations of some androgen metabolites reported in the literature were also included in the calculations, namely: DHT, 5-androstene-3 beta, 17 beta-diol (Ae) and 5 alpha-androstane-3 alpha, 17 beta-diol (Aa). The binding constants for Ae and Aa to TeBG and albumin were also from the literature. Mean values of testosterone were calculated for 11 normal men and expressed as percentages of total: 2.0% was unbound, 53--55% bound to albumin and 43--45% bound to TeBG. For 16 normal women of a fertile age the corresponding values were 1.5%, 36--37% and 62%. For estradiol they were 2.4% 68--70% and 28--30% in the men and 2.0%, 52% and 45--46% in the women. Variations in the concentrations of DHT. Ae and Aa did not influence the free concentrations of testosterone and estradiol to any significant extent. It was furthermore concluded that the androgen metabolites could be omitted from the calculations without affecting the calculated concentrations.