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) clotting, and immediately chilled in ice water. Centrifugation was carried out in the cold, and the extract prepared within four hours. These precautions were intended to reduce in vitro lipolytic action which might cause artifactual elevation of the UFA concentration.Cold storage of normal or lipemic plasma samples for periods of up to four hours has been shown not to result in significant changes of UFA concentrations. To 2 ml. of plasma were added, in order, 1 ml. phosphate buffer of pH 6.0 and molarity 0.2, 1 ml. 5 per cent sodium dodecyl sulfate, 9 ml. saturated aqueous sodium sulfate, and finally a slight excess of anhydrous sodium sulfate powder. The reagents utilized were of the best commercial grade. Five extractions with 2-ml. portions of ethyl ether were carried out as described by Davis. After gentle evaporation of the ether, the residue was taken up in 5 ml. 95 per cent alcohol, a drop of approximately .01 normal sulfuric acid was added, and the solution titrated. The titrant was .02 normal sodium hydroxide (aqueous solution) delivered from a Gilmont ultramicroburette. CO-free air served to stir the solution, and the titration was followed with a Beckman Model G glass electrode pH meter. Titration between the apparent

In a recent publication (1), studies of the concentration of unesterified fatty acid (UFA) in the blood of normal human subjects were reported.The relation of the concentration of unesterified fatty acid to the nutritional state of the subject, together with the observation of arteriovenous differences in unesterified fatty acid concentration, formed the basis for the belief that this lipid fraction constitutes a transport form of fatty acid which can account for the movement of fats from adipose tissue depots into tissues requiring fat as an energy-yielding substrate. Other studies were reported which served to indicate the existence of hormonal mechanisms controlling the concentration of circulating unesterified fatty acid. At the same time, there appeared another publication (2), representing independent investigations with a different analytical method, in which virtually identical observations were reported (with the exception that no studies of arteriovenous differences were included). In view of the potentially great physiologic significance of plasma unesterified fatty acid, it was decided that the above-mentioned studies should be confirmed and extended, and that another analytical procedure, more accurate and convenient than that used earlier, should be developed. The method employed by Dole (2), though extremely convenient and rapid, did not seem suitable for the investigation of arteriovenous differences, since lactate interferes appreciably with the results obtained by that method. The data reported in the present publication were obtained by a procedure which has the advantage of high specificity, although it is appreciably more laborious than the method of Dole. MATERIALS AND METHODSTwo groups of hospitalized patients were utilized as subj ects for this study. The observations on arteriovenous differences were made on patients undergoing cardiac catheterization for the diagnosis of organic heart disease. These subjects were not strictly normal, but they had all received medical treatment as indicated to alleviate their symptoms, and none was used for this work who had clinically demonstrable congestive failure or suffered from any intercurrent metabolic disorder. They were fasting when studied, and were at rest physically, though for the most part there was considerable apprehension concerning the procedure to be carried out. The observations involving sequential samples of blood from the antecubital vein were made on normal conscientious objectors. These young men were hospitalized, had had a complete medical investigation, and were free of disease. They were fasting when studied, at rest physically, and, in general, were not greatly concerned emotionally with the procedure. In the observations involving sequential sampling of venous blood, care was taken to obtain all samples of a series from the same point in the same vein, in order to avoid superposing on the time course of the fatty acid concentration variations related to the fact that different veins might drain tissues whose behavior in ex...

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