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Dr.Kats posted an update
Free-form L-tryptophan IS the "folic acid" precursor needed to make folinic acid
FACTORS AFFECTING THE BIOSYNTHESIS OF FOLINIC ACID BY ESCHERICHIA COLI’
https://journals.asm.org/doi/pdf/10.1128/jb.78.4.488-491.1959
“The best yields of citrovorum factor [aka folinic acid aka leucovorin] were obtained when the medium contained amino acid composition analogous to casein (namely upon the addition of free-form L-tryptophan). Neither folic acid, p-aminobenzoic acid, paminobenzoyl glutamic, nor 2,4, 5-triamino-6- hydroxypyrimidine, enhanced the production of folinic acid.”
