8 Humans need only add fat to their diet to show such lesions. Rats have much higher levels of the enzyme ATPase in the heart, which is important in energy metabolism, so they are very resistant to digitalis drugs used in humans for heart failure. Yet rats differ from humans in many ways that have major effects on cancer research. Rats split carotenoids within the cells lining their intestinal walls using a specific enzyme, thus forming vitamin A. Rats convert most or all dietary carotene to vitamin A. Humans, in contrast, absorb substantial amounts of unchanged carotenoids, and store approximately 15 percent of it in the body. Vitamin C plays vital roles in neutralizing free radicals, preventing cancer and scurvy, and other physiological functions.
Rats synthesize vitamin C in the liver from glucose, using an enzyme called L-gulono oxidase.
Humans do not synthesize vitamin C at all, due to the absence of this enzyme and perhaps also a second enzyme called D-glucurono reductase. Although most animal species can synthesize vitamin C, humans cannot. However, rats are poor predictors of human cancer risk. Experimenters conducting such tests look for the development of cancer in rats. Yet rats are susceptible to different types of cancers from those affecting humans.