A NUMBER of diseases occur in man whose principle therapeutic requirement is supplementation of the diet with pyridoxine (vitamin B6). A variety of interesting mechanisms underlie the need for this supplementation in the different disorders. This paper discusses these mechanisms and how some of them illustrate a new outlook on genetic modifications of biochemical reactions.
Several forms of vitamin B6 occur in biological systems; pyridoxal, pyridoxamine, and pyridoxine are found mainly in the extracellular fluid from where they may be readily transported intracellularly.1 The most important intracellular forms of the vitamin are pyridoxal-5′-phosphate (PLP) (Figure), and pyridoxamine-5′-phosphate. All of the free forms are interchangeable and each can be phosphorylated; the latter forms are also interchangeable. Pyridoxal-5′-phosphate is degraded in the liver by oxidation to 4-pyridoxic acid (4-PA), which is then excreted in the urine. A series of specific enzymes effect these reactions.3-5 Pyridoxal-5′-phosphate is the coenzyme