UNBOUND FREE FATTY-ACID LEVELS IN HUMAN SERUM

Total serum free fatty acids (FFA(t)) levels provide an important meas ure of the physiologic state. Most of the FFA. in serum is bound to al bumin; a small portion, however, is unbound. This study presents the f irst measurements of serum unbound free fatty acid (FFA(u)) concentrat ions. These measurements were made possible by the development of the fluorescence probe ADIFAB (acrylodated intestinal fatty acid binding p rotein). In the present study ADIFAB was shown to provide the correct value of the sum of the [FFA(u)] of each molecular species of long cha in FFA: 1) in aqueous mixtures of FFA and ADIFAB; 2) in mixtures of FF A, human serum albumin, and ADIFAB; and 3) in serum and ADIFAB. Human serum [FFA(u)] were measured in 283 samples from healthy donors and yi elded a mean Value of 7.5 nM with a standard deviation of 2.5 nM. This measured value is as much as 1000-fold smaller than [FFA(u)] values p reviously estimated. The distribution of [FFA(u)] values was found to be invariant with donor gender and age. Average [FFA(u)] values do, as expected, exhibit a small (1.5 nM) but significant (P < 0.001) increa se after overnight fasting. FFA(u) levels were found to be well correl ated with total serum FFA or, equivalently, [FFA(t)]/[HSA]. Given thei r ease and accuracy, and because [FFA(u)] Values increase exponentiall y with [FFA(t)]/[HSA], [FFA(u)] measurements could provide a sensitive method for assessing the physiologic state.