Harper's Illustrated Biochemistry, Volume 1 - Murray R. 1993

Bioenergetics and Metabolism of Carbohydrates and Lipids
Transport and Storage of Lipids
Plasma Lipoprotein Metabolism - Free Fatty Acids (FFA)

Free Fatty acids (non-esterified fatty acids) enter Blood Plasma As a result of the lipolysis of triacylglycerols catalyzed by lipase in adipose tissue, or are formed through the action of lipoprotein lipase on plasma triacylglycerols as they are taken up by Tissues. In blood plasma, FFAs are bound to serum albumin, and their concentration ranges from 0.1 to 2 µeq∙mL^-1 of plasma. They consist mainly of long-chain fatty acids characteristic of adipose tissue (palmitic, stearic, oleic, palmitoleic, linoleic, and other polyunsaturated acids), and to a lesser extent, other long-chain fatty acids. Albumin features several fatty acid-binding sites with varying affinities. Notably, under conditions of adequate Nutrition, the concentration of free fatty acids in the blood remains relatively low. It rises to approximately 0.5 µeq∙mL^-1 following the intestinal absorption of nutrients, and reaches 0.7–0.8 µeq∙mL^-1 during fasting. In cases of uncontrolled Diabetes Mellitus, plasma fatty acid levels can surge up to 2 µeq∙mL^-1.

In most animals, plasma free fatty acid levels drop immediately after a meal and subsequently rise again, whereas in ruminants—whose digestive tracts continuously supply nutrients from the intestine—plasma free fatty acid concentrations remain consistently low.

Free Fatty acids are cleared from the blood very rapidly. It is estimated that during fasting, The oxidation of free fatty acids supplies roughly 25–50% of the energy required to maintain basal metabolic Functions. A fraction of the free fatty acids undergoes Esterification and, as demonstrated by radioisotope analysis, ultimately re-enters metabolic pathways. The respiratory quotient during starvation indicates that The amount of fat broken down significantly exceeds the amount of free fatty acids directly oxidized; this discrepancy is attributed to the oxidation of esterified Lipids derived from blood plasma or stored within various tissues. Lipid oxidation is considered characteristic of cardiac and Skeletal Muscle tissues (with significant lipid reserves having been detected in the latter). A direct correlation exists between the turnover rate of free Fatty Acids and their concentration in the blood.

The concentration of plasma free fatty acids is regulated by The rate of their production in adipose tissue, whereas their uptake by other tissues depends directly on their concentration in blood plasma. The proportion of free fatty acids taken up by specific tissues appears to be largely independent of nutritional status. However, nutritional status does influence the balance between oxidized and esterified fatty acids. For instance, a larger fraction of free fatty acids undergoes oxidation during fasting than under a normal diet.

A fatty acid-binding protein known as Z-protein has been identified in the Cytosol of Cells across numerous tissues. It is believed that, much like serum albumin which facilitates the extracellular transport of long-chain fatty acids, Z-protein mediates their Intracellular Transport.