Molybdenum (Mo) is a trace mineral found in plant protein and in liver. It is necessary to process sulfites and sulfur foods into sulfate. It's also used to make BH4.
Reasons to Suspect
Asthma, crackly joints, arthritis, salicylate sensitivity, sulfur sensitivity, dislike of eggs or other sulfur foods, low BH4 (depression, low dopamine, low thyroid).
The best sources of molybdenum are lentils and other legumes, other plant protein, and presumably liver.
The enzymes that require molybdenum are:
sulfite oxidase - converts sulfite into sulfate
xanthine oxidase - used in purine synthesis, including the purines needed to make BH4
USGS: geographical distribution of Mo