Serine palmitoyltransferase (SPT). The heterodimer formed with SPTLC2 or SPTLC3 constitutes the catalytic core. The composition of the serine palmitoyltransferase (SPT) complex determines the substrate preference. The SPTLC1-SPTLC2-SPTSSA complex shows a strong preference for C16-CoA substrate, while the SPTLC1-SPTLC3-SPTSSA isozyme uses both C14-CoA and C16-CoA as substrates, with a slight preference for C14-CoA. The SPTLC1-SPTLC2-SPTSSB complex shows a strong preference for C18-CoA substrate, while the SPTLC1-SPTLC3-SPTSSB isozyme displays an ability to use a broader range of acyl-CoAs, without apparent preference (By similarity). Required for adipocyte cell viability and metabolic homeostasis (By similarity). H(+) + hexadecanoyl-CoA + L-serine = 3-oxosphinganine + CO2 + CoA H(+) + L-serine + octadecanoyl-CoA = 3-oxoeicosasphinganine + CO2 + CoA H(+) + L-serine + tetradecanoyl-CoA = 3-oxohexadecasphinganine + CO2 + CoA dodecanoyl-CoA + H(+) + L-serine = 3-oxotetradecasphinganine + CO2 + CoA Lipid metabolism; sphingolipid metabolism. Heterodimer with SPTLC2 or SPTLC3. Component of the serine palmitoyltransferase (SPT) complex, composed of SPTLC1, either SPTLC2 or SPTLC3, and either SPTSSA or SPTSSB. The composition of the complex will define the substrate specificity. Interacts with SPTSSA and SPTSSB; the interaction is direct. Interacts with ORMDL3 (By similarity). Interacts with RTN4 (isoform B) (By similarity). Phosphorylation at Tyr-164 inhibits activity and promotes cell survival. Belongs to the class-II pyridoxal-phosphate-dependent aminotransferase family.