A rapid and simple method for the determination of esterified fatty acids and for total fatty acids in blood.

Carboxylic acid esters react with hydroxylamine in alkaline solution to form hydroxamic acids. These acids produce a red to violet reaction with ferric chloride which has been utilized by Feigl (1949) for the microchemical detection of carboxylic acid derivatives. Lipmann and Tuttle (1945) developed a method for the quantitative assay of acylphosphates by allowing hydroxylamine to react with these substances in aqueous solutions at pH 6.0. Hestrin (1949) adapted the reaction for the estimation of short-chain esters by employing alkaline aqueous reagents. Under these conditions esters of higher fatty acids did not react. In order to achieve a reaction of neutral fats and other natural lipids with these reagents, Bauer and Hirsch (1949) employed strictly anhydrous conditions and developed a method for the determination of the total esterified fatty acids in blood serum. Since almost all the fatty acids in blood serum are in esterified form, this method gives results identical with those testing total fatty acids in blood serum. Although the method of Bauer and Hirsch is simpler than most other available methods for blood fat determination, it still involves several time-consuming steps. The blood extracts have to be evaporated to dryness at 600 C. and the evaporation has to be repeated, with the strict exclusion of water. In connexion with work on the uptake of fat from blood serum by tissues in vitro, a convenient and fairly accurate method was required for blood fat estimation. It was learnt by personal information from Oren and Hestrin that higher fatty acid esters react at room temperature with hydroxylamine in alkaline solutions in aqueous alcohol. The presence of alcohol makes the reaction possible and also keeps the long-chain