Mammalian opiate alkaloid synthesis: lessons derived from plant biochemistry

The presence of opiate receptors in mammalian tissues has stimulated the search for endogenous ligands to these receptors and has led to the discovery and characterization of endogenous opioid peptides. However, recent studies have provided evidence for the presence of opiate alkaloids in mammalian tissues and for their endogenous synthesis. The study of their origin and synthetic pathway has been significantly influenced by the early classical biochemical studies performed in plants. This review is a historical account of the use and abuse of opiates, the elucidation of morphine's synthetic pathway in the poppy plant, and the subsequent characterization of its presence in mammalian tissues. Clearly, our understanding of its synthetic pathway and regulation is a reflection of observations originally made in plant biochemistry.