Primary hyperoxaluria types I and 2 (PH1 and PH2) are characterized by an inability to efficiently metabolize glyoxylate as a consequence of functional defects in alanine-glyoxylate aminotransferase and glyoxylate reductase, respectively. Excessive oxalate synthesis results and may ultimately cause renal failure. To date, there are no therapies that can specifically reduce oxalate synthesis. Hydroxyproline metabolism is the only recognized source of glyoxylate that has been identified to date. We hypothesize that the breakdown of dietary and endogenous hydroxyproline contributes to the bulk of the oxalate synthesized in PH1 and PH2 patients. Confirmation of this hypothesis would suggest that the inhibition of hydroxyproline degradation may be a significant therapeutic opportunity for diminishing endogenous oxalate production in PH patients. As a first step toward testing this hypothesis, we have synthesized homogeneously labeled 13C-hydroxyproline and verified that it can be used to follow the degradation of hydroxyproline to oxalate, glycolate, lactate, and glycine using chromatographic techniques coupled to mass detection.
The specific aims of the proposed research are: (1) to quantitate the contribution of hydroxyproline metabolism to endogenous oxalate synthesis in normal human subjects and patients with PH using homogeneously-labeled Hyp, 13C5-hydroxyproline, as a metabolic tracer;(2) to quantitate the contribution of hydroxyproline metabolism to endogenous oxalate synthesis in mouse KO models of PH (Agxt and Grhpr KO) using 13C5-hydroxyproline. Through the quantitation and tracking of the isotopic label within metabolites, we will be able to demonstrate whether or not hydroxyproline degradation contributes to endogenous glycolate, and oxalate synthesis. Moreover, the mouse data will benchmark the contribution of hydroxyproline to urinary glycolate and oxalate in each genetic setting. This latter data will be invaluable for the future testing of therapeutics targeting the unique enzymes of the hydroxyproline degradation pathway.
The synthesis of oxalate, a key component of kidney stones, is influenced by glyoxylate levels. Glyoxylate is produced from hydroxyproline during the normal degradation of collagen within the body and that consumed in the diet. The purpose of this study is to determine whether or not hydroxyproline degradation contributes adversely to the elevated levels of glyoxylate and oxalate production observed in primary hyperoxaluria patients.
|Jiang, Juquan; Johnson, Lynnette C; Knight, John et al. (2012) Metabolism of [13C5]hydroxyproline in vitro and in vivo: implications for primary hyperoxaluria. Am J Physiol Gastrointest Liver Physiol 302:G637-43|