Interferons: from virus inhibitor to modulator of amino acid and lipid metabolism.

Purity of interferons has facilitated definition of pleiotropic biological effects. Alterations that might be suspect with use of impure interferons, such as those occurring in tryptophan and lipoprotein metabolism, have been defined both in vitro and in humans. Reduction in tryptophan contributes to antimicrobial effects for intracellular pathogens and may explain some clinical observations. Decreases in plasma lipoproteins occur rapidly and are of a magnitude similar to cholesterol-lowering drugs used clinically. Alteration in metabolism of amino acids and fats substantially extends the biological effects of a virus-inhibitory protein.