* Chronically elevated homocysteine is infamous for its association with heart disease and may also play a role in memory impairment. It degrades collagen and elastin and damages the lining of blood and lymphatic vessels, via oxidative stress, inflammation, fibrin deposits, etc. Similarly, it compromises the blood brain barrier.
homocysteine-to-methionine step prevents methyl from linking to B12, and thereby impedes methionine formation.
Homocysteine is instead diverted down the transsulfuration pathway to form glutathione.
This pathway transforms homocysteine into sulfur antioxidants and detox agents. And typically removes half the homocysteine from the methylation cycle. But when it goes wrong, it produces toxic sulfur compounds.
Here is an introduction to the chemistry:
vitamin B6 (as P5P), vitamin B3 (as NAD),
Or, lacking required nutrients or energy, or with sulfur accumulation or dietary sulfur overload:
Homocysteine forms cystine (instead of cysteine), excessive ammonia, hydrogen sulfide and other toxic sulfur compounds.
Specifically (enzymes are in parenthesis):
1 Homocysteine + B6 (as P5P*) + Mg (via CBS) yields Cysteine (+ AKG** + Ammonia).
2 Cysteine + serine + selenium + glutamate yields Glutathione.
3 Cysteine + vitamin B3 (as NAD) yields Taurine.
4a Sulfite (from cysteine or food) + molybdenum + B6 (P5P) + oxygen (via SUOX***) yields Sulfate.
4b Without these nutrients, Sulfite produces toxic sulfur compounds.
* B6 metabolism to P5P is supported by AKG, B6, and magnesium. If there is difficulty forming P5P, it is often supplemented directly.
** AKG (alpha-ketoglutaric acid) is a Kreb's energy cycle component. AKG also helps remove ammonia, and form P5P.
Cysteine is the precursor of glutathione, taurine and sulfate.
For information on effects of histadelia on bipolar symptoms, see my book, Natural Healing for Bipolar Disorder.