A nonperturbing 13C nuclear magnetic resonance (NMR) method was used to monitor the equilibrium distribution of carboxyl 13C-enriched fatty acids (FA) between distinct binding sites on human serum albumin, native human lipoproteins, and/or phospholipid model membranes, under conditions that mimic the normal and diabetic human circulation. Two variables pertinent to the diabetic circulation were examined: FA/albumin mole ratio (as elevated in insulin deficiency and/or nephrosis) and pH (as decreased in acidosis). 13C NMR spectra for samples containing carboxyl 13C-enriched palmitate, human serum albumin, and phospholipid vesicles or native lipoproteins (all samples at pH 7.4, 37 degrees C) exhibited up to six carboxyl NMR resonances corresponding to FA bound to distinct binding sites on albumin and nonalbumin components. When the sample FA/albumin mole ratio was 1, three FA carboxyl resonances were observed (182.2, 181.8, and 181.6 ppm; designated peaks beta, gamma, and beta', respectively). These resonances corresponded to FA bound to three distinct high-affinity binding sites on human serum albumin. When the sample mole ratio value exceeded 1, additional carboxyl resonances corresponding to FA bound to phospholipid vesicles (179.0 ppm, peak phi), lipoproteins (180.7 ppm, peak sigma), and lower affinity sites on albumin (183.8 ppm, peak alpha; 181.9 ppm, peak gamma'), were observed. The intensity of peaks phi and sigma increased with increasing mole ratio or decreasing pH. Using Lorentzian lineshape analysis, the relative mole quantities of FA bound to albumin and nonalbumin binding sites were determined. Plots of the fraction of FA associated with nonalbumin components as a function of FA/albumin mole ratio were linear and extrapolated to the abscissa at a mole ratio value of 1. This pattern of FA distribution was observed regardless of the type of nonalbumin acceptor used (phospholipid vesicles, human high- or low-density lipoproteins) or the type of FA used (palmitate, oleate, or stearate), and provided evidence for negative cooperativity for human serum albumin upon binding of 1 mol of FA per mole albumin. These in vitro NMR results suggest that the threshold FA/albumin mole ratio value for alterations in FA distributions in the human circulation may be 1, rather than 3, as previously held. The pathophysiological implications of these findings are discussed.