Conversion of MHPG to vanillylmandelic acid. Implications for the importance of urinary MHPG
P. A. Blombery, I. J. Kopin, E. K. Gordon, S. P. Markey and M. H. Ebert
Deuterium-labelled 4-hydroxy-3-methoxyphenylglycol (MHPG), when
administered intravenously, is rapidly converted to
4-hydroxy-3-methoxymandelic acid (or vanillylmandelic acid [VMA]) or
conjugates of MHPG. Since over half of either racemic D,L-MHPG or the
natural D-MHPG is converted to VMA and since about half of urinary VMA is
derived from MHPG, estimates of the proportion of urinary MHPG derived from
the brain must be revised. The results indicate that only about one fifth
of urinary MHPG is derived from the brain, and clearly urinary MHPG cannot
be used as a valid index of brain norepinephrine metabolism. While these
observations do not alter the value of urinary MHPG as a predictor of
therapeutic response or in subclassifying affective disorders, it is clear
that new research questions must be formulated and appropriate
investigations completed before the relationship of urinary MHPG to
affective disorders is understood.
