New vitamin may be useful cholesterol-lowering supplement
important role in processes such as gene regulation, metabolism and
ageing.
They say that supplementation with the nutrient, identified in whey, could offer an alternative to niacin, which fights cholesterol but has uncomfortable flushing effects in patients.
The work, published in the 14 May issue of Cell (vol 117, 495-502) defines a new metabolic route to an important small molecule in cells called nicotinamide adenine dinucleotide (NAD).
The molecule is a coenzyme that both partners with enzymes and is consumed by others. Niacin, or vitamin B3, is a mixture of the NAD precursors nicotinic acid and nicotinamide, which can help control cholesterol.
Study author Dr Charles Brenner, associate professor of genetics and biochemistry at Dartmouth Medical School in the US, was studying an enzyme involved in NAD synthesis that was similar to an enzyme implicated in cancer development. His investigation found that in yeast without the enzyme, every known NAD biosynthetic pathway was shut down, so in theory, the cells should die; no vitamins or supplements were known to keep the cells alive.
But another NAD precursor, nicotinamide riboside, thought to be a vitamin form of NAD only in certain bacteria, was found to act as a vitamin in the yeast, preventing death. Yeast has similarities to mammalian cells, and a gene or enzyme in yeast is likely to have an equivalent in humans.
So after finding the genes and enzymes in yeast, the researchers identified the genes and enzymes in humans responsible for this vitamin conversion pathway.
Their findings upend some long-established assumptions about NAD. To reinforce the vitamin premise, Brenner's team looked for the compound in a food and found it in milk whey.
"We cloned the yeast nicotinamide riboside kinase, and validated it genetically by knocking it out and seeing that the vitamin no longer supported growth," said Brenner. "We then identified two human nicotinamide riboside kinases and showed that all three have the biochemical specificity for nutrient and prodrug [precursor] activation and that all three work in vivo."
Nicotinamide riboside may be a useful nutrient for certain metabolic disorders, possibly as an alternative to niacin for cholesterol-lowering, the researchers suggest.
