Chemical and biochemical aspects of various food constituents were investigated in detail to elucidate their physiological functions and nutritional significance. 1) A nitrogen-protein conversion factor of 4.0 was employed to calculate the crude protein contents of cultivated mushrooms. 4-Aminobenzoate hydroxylase newly isolated from Agaricus bisporus was shown to be useful for selective determination of 4-aminobenzoate in pancreatic function diagnosis. 2) N^G-Methylarginine derived from proteins methylated post-translationally was demonstrated to be actively metabolized by a newly identified enzyme, dimethylargininase, suggesting that this metabolic system might take part in the regulation of nitric oxide synthesis by removing methylated arginine as an endogenous blocker. The major origin of endogenous dimethylamine as a possible precursor of potent carcinogens in animals was demonstrated to be a degradation product of N^G-monomethylarginine by dimethylargininase. 3) The major food allergenic protein occurring in soybean was identified as a 34-kDa oil-body-associated protein, which is classified as a member of the papain-like thiol proteinase family highly homologous to house dust mite allergen, Der p I, the most frequent causative allergen for young patients with atopic dermatitis.