(S)-norcoclaurine is the central intermediate in benzylisoquinoline alkaloid biosynthesis

Feeding experiments with (S)-[1-13C]-norcoclaurine demonstrate that this trihydroxylated precursor is specifically incorporated into protoberberine, aporphine and benzophenanthridine alkaloids in cell suspension cultures, as well as into pavine and benzophenanthridine alkaloids in whole plants. The rates of incorporation ranged from 2.5 to 36%. This reveals that tyrosine is metabolized to dopamine and p-hydroxyphenylacetaldehyde which then condense to form norcoclaurine, thus explaining the lack of incorporation of DOPA or dopamine into the benzylic portion of reticuline derived alkaloids.