Effect of Dietary Protein on the Enzyme from Rat and Human Intestine Which Converts β-Carotene to Retinal

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Abstract
The enzyme which oxidizes β-carotene to retinal was obtained from the mucosa of rat small intestine and assayed. Values for rats on 5, 10, 20, and 40% protein intake, respectively, were 1.26 ± 0.20, 3.14 ± 0.36, 2.10 ± 0.28, and 1.84 ± 0.20 µmoles (× 10-5) of retinal per milligram of protein from the supernatant fraction; the values were obtained with optimum substrate concentrations. Irrespective of sex or of pair vs. ad libitum feeding, rats fed 10% protein had also the highest enzyme activity per total mucosa and per DNA. The high activity of animals fed the 10% protein diet could not be explained by postulating the presence of an activating cofactor (tested by adding purified enzyme from an animal fed 40% protein to the supernatant fraction of a 10% protein animal). Protein synthesis in the rats fed 10% protein was highest, and these rats also showed the heaviest polyribosome aggregates when compared to 5, 20, and 40% protein rats. Neither β-carotene nor retinol depletion, or excess feeding of these two substances, greatly affected the enzyme. The specific activity of the enzyme obtained from the duodenum of human volunteers was similar to that of the rat. A protein-free diet consumed by human subjects for 8 to 9 days had no effect on the activity of the enzyme.