Selective mobilization of fatty acids from white fat cells: Evidence for a relationship to the polarity of triacylglycerols

Fatty acids are selectively released from white fat cells in accordance with well-defined rules relating their molecular structure and their mobilization rate, emphasizing the possible role of their physicochemical properties. Lipolysis is widely reported to work for conditions where only small amounts of substrate are available. We hypothesize that the preferential hydrolysis of a substrate fraction enriched in the most polar triacylglycerols (TAGs) reflects the pattern of selective fatty acid mobilization. Rat adipose tissue was first manipulated by dietary means to obtain a wide spectrum of fatty acids, Fat cell TAGs were separated into eight fractions according to polarity by liquid-liquid partition chromatography and their fatty acid proportions and compositions were determined by GLC. In the most polar TAG fractions, the relative enrichment of fatty acids (percentage in a TAG fraction divided by percentage in total TAGs) increased with the number of double bonds for a given chain length, whereas it decreased with increasing chain length for a given degree of unsaturation. The relative enrichment of highly mobilized fatty acids (16-20 carbon atoms and four or five double bonds) was very high (more than 2.5) in the most polar TAG fractions, whereas that of weakly mobilized fatty acids (20-24 carbon atoms and no or one double bond) was very low (less than 0.5). The relative enrichment of moderately mobilized fatty acids (comprising all the others) was close to unity. Our study shows that the relative enrichment of fatty acids in the most polar adipose tissue TAGs is consistent with their relative mobilization rates. This supports our hypothesis and raises the possibility that the molecular species of TAGs might be one of the regulating factors.