The aqueous humor of rabbit contains high concentrations of pyrroline-5-carboxylate.

Accumulating evidence suggests that the interconversion of proline and pyrroline-5-carboxylate (P5C) transfers oxidizing or reducing potential within or between cells. We report here that rabbit aqueous humor contains P5C at a concentration at least 20 times that found in rabbit plasma or in biologic fluids from other animals, and, in isolated lens, P5C stimulates the activity of the pentose phosphate shunt. However, aqueous humor in other species contains P5C in the same range of concentrations as their respective plasma. The high P5C concentration in rabbit aqueous humor may be due to comparatively low levels of P5C reductase activity in ocular tissues, especially the cornea. The levels in lens epithelium, though lower in the rabbit, nevertheless could mediate the transfer of oxidizing potential from P5C and stimulate the pentose-phosphate shunt. These results suggest that the eye may serve as a convenient model for the study of proline and P5C-related transfer of reducing or oxidizing potential between tissues.