Unesterified fatty acid in human blood plasma.

The existence of small quantities of unesterified fatty acid in blood plasma has been known for some years. In particular, the finding of unesterified fatty acid (hereafter abbreviated UFA) in albumin fractions prepared from fresh human plasma by the methods of Cohn, Hughes, and Weare (1), and the report of Davis (2) that fresh serum yielded small quantities of UFA, indicate that UFA actually exists as such in plasma and is not a laboratory artifact. In connection with the investigations into the mechanism of lipid transport being carried forward in this laboratory, it had been found (3, 4) that a principal product of lipemia clearing in vitro is unesterified fatty acid. Study of the relationship of physiologic clearing in vivo to the concentration of circulating UFA therefore suggested itself, and led to the present investigation. It will be observed that the term "unesterified fatty acid" is used in preference to "free fatty acid" in spite of the widespread acceptance of the latter in fat and oil chemistry. The choice is due to the evidence (5-7) that higher fatty acids are firmly, though reversibly, bound to serum albumin and at times to other plasma proteins when proteins and fatty acids exist together in aqueous solution. The properties of the fatty acid ions, such as surface activity, are therefore not in evidence when small quantities of these substances are present in serum or plasma (8). We suggest that the term "free" be reserved to describe the fraction of unesterified fatty acid not bound to protein, a fraction which is not measurable by present techniques, but which may later prove to be of biochemical significance.