d-limonene mechanistic data and risk assessment: absolute species-specific cytotoxicity, enhanced cell proliferation, and tumor promotion.

d-Limonene produces tumors only in the kidneys of male rats in association with hyaline-droplet nephropathy, which is due to the accumulation of the rat-specific, low molecular weight protein alpha(2u)-globulin in the P2 segment cells of renal proximal tubules. Human urine contains no alpha(2u)-globulin and, compared with the male rat, much less protein and almost no low molecular weight protein. Genotoxicity tests for d-limonene are negative, and the mechanism of tumorigenesis involves tumor promotion and enhanced cell proliferation. There is no risk of cancer for humans from d-limonene, since the binding of d-limonene to alpha(2u)-globulin would not occur.