MUTAGENIC ACTIVITY OF PEPTIDES AND THE ARTIFICIAL SWEETENER ASPARTAMEAFTER NITROSATION

Naturally occurring dipeptides, cholecystokinine (CKK, a tetrapeptide hormone) and the artificial sweetener aspartame were nitrosated for 10 -30 min with 40 mM-nitrite (pH 3.5, 37-degrees-C), and the resultant p roducts examined for mutagenicity in Salmonella typhimurium TA100. Spe cific mutagenicities (net revertants per mumol precursor) spanned four orders of magnitude, with CCK being the most potent precursor (4700 r evertants/mumol) followed by tryptophyl-tryptophan (Trp-Trp; 1000 reve rtants/mumol). Aspartame and glycyl-Trp (Gly Trp) had intermediate act ivity (300 revertants/mumol), while Gly-Gly and methionyl-methionine w ere only weakly mutagenic (20 and 12 revertants/mumol, respectively). The dipeptides of aspartic acid, phenylalanine and tyrosine had no det ectable mutagenicity (limits of detection 0.5, 40 and 5 revertants/mum ol, respectively). Kinetic studies with aspartame and Gly-Trp suggeste d that the mutagenic products arose primarily from nitrosation of the primary amine rather than the amide or indole group. The mutagenicitie s of nitrosated aspartame and Gly-Trp were higher in TA100 than in TA9 8, and higher without than with enzymatic activation (S-9 mix) in both strains. The time-course study of Trp-Trp nitrosation showed the prod uction of at least two mutagens: a potent but unstable mutagenicity wa s seen at very short nitrosation times and a more stable but weaker ef fect was obtained after more than 60 min of nitrosation. Not only the absolute specific mutagenicity but also the nitrite dependence of the nitrosation reaction and the stability of the nitroso product must be taken into account in determining the risk posed by endogenous nitrosa tion of foods in the human stomach. Under stomach conditions, nitrosat ion of the side-chains of certain Trp peptides would be expected to co ntribute more to the endogenous burden of nitrosated products than nit rosation of aspartame or Gly peptides.