2.4. Polyunsaturated fatty acids

Most vertebrates are capable of synthesizing fatty acids containing either no (saturated) or one (monounsaturated) carbon-carbon double bond per molecule, but they cannot produce fatty acids containing two or more double bonds, or PUFAs. The two principal dietary essential fatty acids (EFAs) in vertebrates are linoleic acid (18:2n-6 or 18:2) and α-linoleic acid (18:3n-3) (Noble 1981, Innis 1991, Bruckner 1992). These fatty acids are not interconvertible but they can be further elongated and desaturated by the same enzyme systems into their respective n-6 and n-3 long-chain PUFA derivatives (Bruckner 1992). The principal EFAs and their derivatives are important for the foetal growth and development of young animals by being constituents of cellular membranes (Innis 1991, Bruckner 1992). The n-6 PUFAs possess the ability to eliminate pathogenic bacterial or microbial activity (Bruckner 1992) while very-long chain n-3 PUFAs are vital for the normal development and function of vision and brain (Neuringer & Connor 1986). Certain C20-PUFAs or eicosanoids provide the precursors for prostaglandins and leukotriens that have various physiological functions in the body (Bruckner 1992).

It has long been assumed that lipolysis produces fatty acids in the proportions in which they occur in the TAGs of the adipocytes (Spitzer et al. 1966), or are uptaken from the diet (Ekstedt & Olivecrona 1970). Recent studies have shown that proportionally more PUFAs than saturated fatty acids are released from adipose tissues during lipolysis (Gavino & Gavino 1992, Raclot & Groscolas 1993, Raclot et al. 1995, Connor et al. 1996). In addition, the shorter chain fatty acids (C14-C16) with one double bond are more likely to be hydrolysed than the longer-chain monounsaturated fatty acids. The biochemical mechanism for the preferential mobilization of unsaturated fatty acids is not known. One explanation that has been presented is that TAGs enriched by PUFAs tend to accumulate in the periphery of the TAG droplets and are therefore most susceptible to lipase action (Raclot & Groscolas 1995). So far, there is no evidence to indicate whether all adipocyte or adipose tissue types behave similarly to white adipocytes. However, site-specific differences in the release of unsaturated fatty acids from adipocytes near lymph nodes have been shown (Mattacks & Pond 1997), and there is evidence for the involvement of these fatty acids in immune responses (Pond & Mattacks 1998).