FIGURE 1 Sources of blood oxalate.
From the following article:
Oxalate in renal stone disease: the terminal metabolite that just won't go away
Susan R Marengo and Andrea MP Romani
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Figure 1. Sources of blood oxalate.
(A) Oxalate absorption along the intestinal tract is believed to be mediated by three oxalate-transporting members of the solute carrier family 26A (SLC26A3, SLC26A6, SLC26A7).16, 20, 34 Secretion into the intestinal tract might also occur.24, 25 Only free oxalate can be absorbed by the intestinal epithelium. The amount of free oxalate is affected both by the presence of other ions, such as calcium—which binds to oxalate—and unabsorbed lipids—which indirectly increase the amount of free oxalate by binding to calcium.15, 16, 28 Absorption of oxalate is also affected by the presence of gut bacteria, such as Oxalobacter formigenes, that degrade oxalate to carbon dioxide and formate.16, 32 (B) In the systemic circulation, erythrocytes synthesize oxalate from glyoxylate;17 in addition, oxalate enters erythrocytes via the band 3 anion transport protein (SLC4A1 or AE1).18 Ascorbate (vitamin C) in the blood can be metabolized to oxalate.41 (C) In the liver, oxalate is synthesized from glyoxylate by lactate dehydrogenase and released into the blood. The major pathways controlling the production of glyoxylate have not yet been identified, although the degradation of hydroxyproline and the oxidation of glycolate by glycolate oxidase are two sources of glyoxylate.35, 37 Under normal conditions, most glyoxylate is metabolized to glycine by alanine–glyoxylate aminotransferase or reduced to glycolate by D-glycerate dehydrogenase. Defects in the genes encoding these two enzymes characterize primary hyperoxaluria types I38 and II,39 respectively. (D) Unexcreted oxalate is stored in structural tissues and soft organs. Abbreviations: AGT, alanine–glyoxylate aminotransferase; DGDH, D-glycerate dehydrogenase; GOX, glycolate oxidase; LDH, lactate dehydrogenase; Ox, oxalate; PH-I, primary hyperoxaluria type I; PH-II, primary hyperoxaluria type II; SLC26A3, solute carrier family 26 member 3; SLC26A6, solute carrier family 26 member 6; SLC26A7, solute carrier family 26 member 7; SLC4A1, band 3 anion transport protein.