The measurement of total fatty acid in serum.

A review of the four most popular ap­ proaches to the measurement of total serum fatty acid is presented. Hydroxylam ine, cop­ per soap, and sulfophospho-vanillin methods are sum marized briefly, w hile acidim etric tech­ niques are considered in som ewhat greater de­ tail. T he highly sensitive acidim etric m ethod of M assion and Seligson is presented. In this technique, serum is saponified in alcoholic KOH. M ineral acid is added to precipitate the fatty acid m oieties w hich are extracted into petroleum ether. F ollow ing solvent evapora­ tion, a buffered solution o f m ethyl red indi­ cator is added. A shift of indicator color re­ lated to the concentration of fatty acid in the standards or the serum sam ple is produced and quantitated spectrophotometrically. The m ethod is highly accurate, lacking interfer­ ence, semi-micro, rapid, and versatile. Until about 10 to 15 years ago, precise chemical measurements of the various com­ ponents of serum lipid were relatively dif­ ficult. By contrast, the quantitation of total serum lipid by gravimetric means had been well established for over two decades. As cholesterol and phospholipid methods were improved and became more popular, the necessity for total lipid measurement waned. However, a complete lipid profile of the patient seemed to demand a total lipid value as long as triglyceride concen­ tration was not provided. W ith the advent of good triglyceride methodology, the need for the measure­ ment of total serum lipid has been eclipsed. However, it is possible that an easily stan­ dardized, precise, rapid procedure which accurately reflects the total lipid content might still prove to be clinically useful. Fatty acid constitutes approximately 60 percent of the total serum lipid. One per­ cent of the fatty acid is bound loosely to albumin, while triglycerides contain 25 percent, phospholipids contribute 20 per­ cent and cholesterol esters are the source of the remaining 14 percent. “Free” choles­ terol is the only lipid fraction not asso­ ciated with a fatty acid group. Because of this wide distribution, the accurate mea­ surement of total serum fatty acid is an excellent method of estimating total serum lipid. M any approaches to the measurement of total fatty acid have been developed and are outlined by Henry.21 Four w ill be con­ sidered: hydroxylamine methods, copper soap methods, sulfophospho-vanillin meth­ ods and acidimetric methods. H ydroxylam ine M ethods Feigl’s15 spot test for carboxylic acids and esters is the basis of the hydroxylamine approach. In this method, esters are warmed in an alkaline hydroxylamine me­ dia to produce hydroxamic acid, which forms a red to lavender complex with fer­ ric ion in acid solution. Following the de­ velopment of a quantitative measurement