Mammalian opiate alkaloid synthesis: Lessons derived from plant biochemistry

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 Clearly, our understanding of its synthetic pathw ay and regulation is a reflection of observations originally made in plant biochemistry.