Vanadium can be found in certain organisms in extremely high concentrations. In most cases, these are marine animals like sea squirts, tunicates (e.g. Tunichrome B-1) or fungi (e.g. Amavadin in fly agaric).
Although vanadate inhibits phosphate-dependent enzymes (structural analogy VO43- and PO43-), it stimulates glucose metabolism.
Chromium(VI) compounds are stongly mutagenic and cancerogenic, but the metal in oxidation state +III is an essential trace element. Remarkably, it is responsible for the optimal function of insulin in the glucose tolerance factor.