Biochemistry Metabolism of Purines and Pyrimidines 594d2516 Rate limiting step in porphyrine synthesis - A ALA dehydratase B ALA synthase C UPG decarboxylase D Ferrochelatase High-Yield Explanation Ans. is 'b' i.e., ALA synthase o See above explanation. Heme is the most important porphyrin (porphyrin III) in the body.