The results of our previous study on heterologous expression in Escherichia coli of the gene desD, which encodes Spirulina Δ6 desaturase, showed that co-expression with an immediate electron donor—either cytochrome b 5 or ferredoxin—was required for the production of GLA (γ-linolenic acid), the product of the reaction catalyzed by Δ6 desaturase. Since a system for stable transformation of Spirulina is not available, studies concerning Spirulina-enzyme characterization have been carried out in heterologous hosts. In this present study, the focus is on the role of the enzyme’s N- and C-termini, which are possibly located in the cytoplasmic phase. Truncated enzymes were expressed in E. coli by employing the pTrcHisA expression system. The truncation of the N- and C-terminus by 10 (N10 and C10) and 30 (N30 and C30) amino acids, respectively, altered the enzyme’s regioselective mode from one that measures from a preexisting double bond to that measuring from the methyl end of the substrate.