Alkaloids and Plant Metabolism V. THE DISTRIBUTION AND FORMATION OF TYRAMINE METHYLPHERASE DURING GERMINATION OF BARLEY *

Barley offers several advantages as a model system for studies of the role of alkaloids in plant metabolism. Two distinct classes of alkaloids are synthesized by analogous yet separate mechanisms: N-methylated tyramine (1) and N-methylated indole derivatives (2). Because these alkaloids are made by young plants rather than by mature or flowering plants, large quantities of plant material can be obtained in short times. Hordenine (N , N-dimethyltyramine) has been known for more than 50 years as the predominant alkaloid in roots of germinating barley (3). Since the pioneering work of Raoul (4), a number of investigators have studied the distribution and biosynthesis of this tyramine derivative. Hordenine and its precursor, N-methyltyramine, are absent from barley seeds before germination, but appear in the roots during the first day of germination. The alkaloid level in the roots reaches a maximum within 3 to 9 days; the timing of this peak is reproducible for a given barley variety grown under constant experimental conditions. After the first week, hordenine and N-methyltyramine concentrations slowly decrease until only traces remain 1 month after germinat,ion (5-7). Raoul suggested that hordenine is synthesized by the successive N-methylation of tyramine. However, this investigator was unable to detect the presence of N-methyltyramine in barley roots. The identification of N-methyltyramine in the roots of certain barley varieties was accomplished by Kirkwood and Marion (8). The biosynthetic pathway postulated by Raoul was confirmed by the isolation of labeled N-methyltyramine and hordenine 4 days after radioactive tyramine had been fed to the roots of intact barley seedlings (9). These workers further showed that phenylalanine (10) and tyrosine (11) are more remote precursors of N-methyltyramine and hordenine. The stimuli that initiate hordenine synthesis and stop further accumulation are not known. It is possible that these changes are associated with maturation or differentiation or both. The present work is part of a project that has as its goal the elucidation of the mechanisms controlling these changes. It is hoped that the results will give some insight into the general mechanisms controlling plant metabolic processes. Tyramine methylpherase, the enzyme that catalyzes the N-methylation of tyramine by ( -)-S-adenosyl-L-methionine, has been characterized in a previous publication (12). In the present work we shall trace the level of this enzyme and the related N-methyltyramine