Serine palmitoyltransferase (SPT) (PubMed:28100772). 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. The SPTLC1-SPTLC2-SPTSSB complex displays a strong preference for C18-CoA substrate, while the SPTLC1-SPTLC3-SPTSSB isozyme has the ability to use a broader range of acyl-CoAs (By similarity). Required for adipocyte cell viability and metabolic homeostasis (PubMed:28100772). 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) (PubMed:26301690). Expressed in a variety of tissues. Highest expression in brain, kidney and liver (PubMed:21994399). Expressed in brown and white adipose tissues (PubMed:27818258). Highly expressed after birth, expression decreases 2 weeks after birth and is maintained until, at least, 18 months. Expression levels at protein level increase upon high-fat diet. mRNA levels remain unchanged. Phosphorylation at Tyr-164 inhibits activity and promotes cell survival. Knockout are lethal at embryonic stage (PubMed:28100772). Conditional knockouts specific to the adipose tissue develop adipose tissue but exhibit a striking age dependent loss of adipose tissue accompanied by evidence of adipocyte death, increased macrophage infiltration and tissue fibrosis. They also have elevated fasting blood glucose, fatty liver and insulin resistance. They show a significant reduction of total sphingomyelin levels in the adipose tissue (PubMed:28100772). Belongs to the class-II pyridoxal-phosphate-dependent aminotransferase family.