Cytosolic rat HSL is phosphorylated on serine residues 659 and 660 by protein kinase A catalytic subunit (Anthonsen et al. 1998; Su et al. 2003). Three isoforms of protein kinase A are known, but with no known differences in substrate specificity or tissue specific expression patterns, so a generic PKA (with all three forms as instances) is annotated as the catalyst of this reaction. Other serine residues in HSL can be phosphorylated both in vitro and in vivo, and while these other phosphorylations appear not to affect triacylglycerol hydrolysis by HSL directly, they may affect the efficiency with which serines 659 and 660 themselves are phosphorylated, or affect the efficiency with which HSL is translocated to cytosolic lipid particles (Holm et al. 2000).

Phosphorylation of human HSL has not been studied in detail, so the human reaction is inferred from the well-studied rat one. By BLAST alignment, human HSL residues 649 and 650 correspond to rat serines 659 and 660.