succinyl-CoA + glycine => delta-aminolevulinate + CoA SH + CO2

Mitochondrial ALAS (delta-aminolevulinate synthase) catalyzes the reaction of succinyl-CoA and glycine to form delta-aminolevulinate, CoA SH and CO2. ALAS occurs in two isoforms. ALAS1 is widely expressed in the body though particularly abundant is liver. its expression can be induced by steroids and other substrates of cytochrome P450-mediated reactions, and repressed by hemin. ALAS2 is expressed at high levels in erythroid precursor cells. Both ALAS1 and ALAS2 are active as dimers, with each subunit polypeptide bound to a molecule of pyridoxal phosphate. The two proteins are encoded by separate genes; cDNAs corresponding to both have been cloned (Riddle et al. 1989).

线粒体ALAS（delta-氨基乙酰丙酸合酶）催化琥珀酰辅酶A与甘氨酸的反应，生成delta-氨基乙酰丙酸、辅酶A-SH和二氧化碳。ALAS存在于两种同工酶形式中。ALAS1在体内广泛表达，尽管在肝脏中含量尤为丰富；其表达可被类固醇及其他细胞色素P450介导反应的底物诱导，并由血红素抑制。ALAS2在红系前体细胞中高水平表达。ALAS1和ALAS2均以二聚体形式活跃，每个亚基多肽与一分子吡哆醛磷酸结合。这两种蛋白质由不同的基因编码；与两者相对应的cDNA已被克隆（Riddle等，1989年）。