Metabolism of [13C5]hydroxyproline in vitro and in vivo: implications for primary hyperoxaluria

Hydroxyproline (Hyp) metabolism is a key source of glyoxylate production in the body and may be a major contributor to excessive oxalate production in the primary hyperoxalurias where glyoxylate metabolism is impaired. Important gaps in our knowledge include identification of the tissues with the capacity to degrade Hyp and the development of model systems to study this metabolism and how to suppress it. The expression of mRNA for enzymes in the pathway was examined in 15 different human tissues. Expression of the complete pathway was identified in liver, kidney, pancreas, and small intestine. HepG2 cells also expressed these mRNAs and enzymes and were shown to metabolize Hyp in the culture medium to glycolate, glycine, and oxalate. [(18)O]- and [(13)C(5)]Hyp were synthesized and evaluated for their use with in vitro and in vivo models. [(18)O]Hyp was not suitable because of an apparent tautomerism of [(18)O]glyoxylate between enol and hydrated forms, which resulted in a loss of isotope. [(13)C(5)]Hyp, however, was metabolized to [(13)C(2)]glycolate, [(13)C(2)]glycine, and [(13)C(2)]oxalate in vitro in HepG2 cells and in vivo in mice infused with [(13)C(5)]Hyp. These model systems should be valuable tools for exploring various aspects of Hyp metabolism and will be useful in determining whether blocking Hyp catabolism is an effective therapy in the treatment of primary hyperoxaluria.