Involved in ceramide synthesis. In vitro, isoform 3 stimulates the production of C16-, C18- and C20-ceramides, isoform 1 slightly increases the levels of C18- and C20-ceramides, while isoform 2 exhibits only minimal activity. May interfere with adipogenesis by stimulating ceramide synthesis. octadecanoyl-CoA + sphing-4-enine = CoA + H(+) + N-octadecanoylsphing-4-enine eicosanoyl-CoA + sphing-4-enine = CoA + H(+) + N-eicosanoyl-sphing-4-enine hexadecanoyl-CoA + sphing-4-enine = CoA + H(+) + N-hexadecanoylsphing-4-enine Each isoform has a distinct expression pattern. Isoform 1 is highly expressed in brain. Isoform 2 is expressed at low levels, if any, in all analyzed tissues, with slightly higher levels in testis. Isoform 3 is expressed at very high levels in testis and, at lower levels, in white adipose tissue. In epididymal fat, isoform 3 is expressed at higher levels in obese mice compared with lean mice. By contrast, isoform 1 and 2 levels are significantly lower in obese mice compared with lean mice. Up-regulated during adipogenesis. Strongly up-regulated by PPARG. Knockout mice exhibit a significant reduction of the cone photoreceptor light responses, thinning of the outer nuclear layer, and loss of cone photoreceptors across the retina compared with wild-type animals (PubMed:33077892). Knockout male mice are fertile (PubMed:33077892).