Hydroxyproline Metabolism and Oxalate Synthesis in Primary Hyperoxaluria

Abstract

Endogenous oxalate synthesis contributes to calcium oxalate stone disease and is markedly increased in the inherited primary hyperoxaluria (PH) disorders. The incomplete knowledge regarding oxalate synthesis complicates discovery of new treatments. Hydroxyproline (Hyp) metabolism results in the formation of oxalate and glycolate. However, the relative contribution of Hyp metabolism to endogenous oxalate and glycolate synthesis is not known. To define this contribution, we performed primed, continuous, intravenous infusions of the stable isotope [N,C]-Hyp in nine healthy subjects and 19 individuals with PH and quantified the levels of urinary C-oxalate and C-glycolate formed using ion chromatography coupled to mass detection. The total urinary oxalate-to-creatinine ratio during the infusion was 73.1, 70.8, 47.0, and 10.6 mg oxalate/g creatinine in subjects with PH1, PH2, and PH3 and controls, respectively. Hyp metabolism accounted for 12.8, 32.9, and 14.8 mg oxalate/g creatinine in subjects with PH1, PH2, and PH3, respectively, compared with 1.6 mg oxalate/g creatinine in controls. The contribution of Hyp to urinary oxalate was 15% in controls and 18%, 47%, and 33% in subjects with PH1, PH2, and PH3, respectively. The contribution of Hyp to urinary glycolate was 57% in controls, 30% in subjects with PH1, and <13% in subjects with PH2 or PH3. Hyp metabolism differs among PH types and is a major source of oxalate synthesis in individuals with PH2 and PH3. In patients with PH1, who have the highest urinary excretion of oxalate, the major sources of oxalate remain to be identified.

Dietary Oxalate Intake and Kidney Outcomes

“…In humans, oxalate is a terminal metabolite that is the end product of endogenous metabolism by the liver. Its immediate precursor is glyoxylate, which is either a product of hydroxyproline catabolism or oxidation of glycolate [7,29,30]. Glyoxylate is in turn trans-aminated by alanine glyoxylate aminotransferase (AGT) to form pyruvate and glycine or converted back to glycolate by glycolate/hydroxy-pyruvate reductase (GRHPR).…”

Dietary Oxalate Induces Urinary Nanocrystals in Humans

“…Lastly, the pellet was air dried inside the hood for 2 days prior to weighing and visualizing under a polarized light microscope. The concentration of oxalate in the crystals was determined using IC-MS. 37 Calcium phosphate (CaP; hydroxyapatite) crystals, Calcium green (hexapotassium salt), Fluo-4 pentapotassium salt (Fluo-4), and gold nanoparticles were all purchased from Millipore Sigma (St. Louis, MO). Fluo-4 AM and Calcein Ultragreen AM were purchased from AAT Bioquest (Sunnyvale, CA).…”

“…IC-MS was performed to measure oxalate levels as previously described. 37,38 Pellets harvested from urine following centrifugation were washed with absolute ethanol and centrifuged (4000 RPM for 10 minutes at RT) twice to remove traces of urine and cellular debris (Step 5, Figure 1). Pellets were either resuspended in absolute ethanol prior to NTA; Step 6) or dried with argon gas and resuspended in 0.5 M HCl before determining oxalate, calcium, and phosphate concentrations (steps 7-9, Figure 1).…”

Lactate dehydrogenase 5: identification of a druggable target to reduce oxaluria

“…In PH1, treatment with the AGT1 cofactor pyridoxal phosphate (vitamin B6) has been shown to increase enzymatic activity and decrease oxalate production (14). There is also research to support a diet low in hydroxyproline for PH2 and PH3 (15). For patients with PH, liver transplantation (or combined liverkidney if the patient has endstage renal disease) restores normal oxalate metabolism (11).…”