ABSTRACT
Both of these pathways require two desaturation steps and a single two-carbon elongation reaction. The results of several different types of studies, during the past 40 years have supported, but not conclusively proven that the endoplasmic reticulum contains position-specific ∆6-and ∆5-desaturases. First, when liver microsomes were incubated with mixtures of PUFA, with their first double bond at different positions, the composition of the products was consistent with the presence of position-specific desaturases (7). For example, the desaturation of 9,12-18:2 decreased when 9,12,15-18:3 was included in the incubation suggesting that a single ∆6 desaturase accepted both substrates. Second, when mixtures of different PUFA were fed to rats, the composition of esterified PUFA also suggested the presence of different desaturases (8). Third, several cell lines were able to introduce a double bond at position-5, but not at position-6, when appropriate PUFA were used as substrates (9,10), thus suggesting the presence of position-specific ∆6 and ∆5 desaturases.
