A 55-year-old man with chronic alcoholism and poor diet develops bilateral symmetrical dermatitis on sun-exposed skin, diarrhea, and dementia. The condition is caused by deficiency of niacin (vitamin B3). Pellagra can also result from dietary tryptophan deficiency because tryptophan can be converted to niacin. Approximately how many milligrams of tryptophan are required to synthesize 1 mg of niacin, and which vitamin is an essential cofactor for this conversion?

• A 60 mg tryptophan per 1 mg niacin; pyridoxal phosphate (B6) ✓
• B 6 mg tryptophan per 1 mg niacin; riboflavin (B2)
• C 20 mg tryptophan per 1 mg niacin; cobalamin (B12)
• D 100 mg tryptophan per 1 mg niacin; thiamine (B1)

Explanation

The conversion of tryptophan to niacin via the kynurenine pathway has an efficiency of approximately 60:1 (60 mg tryptophan to 1 mg niacin equivalent), which is why niacin requirements are expressed as niacin equivalents (1 NE = 1 mg niacin or 60 mg tryptophan). Pyridoxal phosphate (PLP, vitamin B6) is an essential cofactor for kynureninase, a key enzyme in this pathway; B6 deficiency therefore compounds niacin deficiency. This also explains why pellagra is part of the carcinoid syndrome — tumor tryptophan consumption for serotonin synthesis depletes tryptophan available for niacin synthesis.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.