mc-1 negatives? vitamin b6 is cheap? pyridoxal-5-phosphate quickly gets metabolized and would reach steady state levels with its substrates, and the excess would get excreted? <A HREF="http://www.genome.ad.jp/kegg/pathway/map/map00750.html" target="_blank">http://www.genome.ad.jp/kegg/pathway/map/map00750.html</A>