300--400 mg of heme is synthesized daily in the organism. Heme is the prosthetic groups of hemoglobulin, cytochromes and other hemoproteins. Major sites of heme synthesis are bone marrow and liver. At least 7 enzymes and intermediary products are involved in the sequence of reactions that lead from glycine and succinyl coenzyme A to heme. delta-Aminole-vulinic acid synthetase, the first enzyme in the sequence of reactions, is rate-limiting for heme formation and important regulatory events are directed at this enzyme.