The excretion of carbonic anhydrase isozymes CA I and CA II in the urine of apparently healthy subjects and in patients with kidney disease

Detection of Intravascular Hemolysis in Newborn Infants Using Urinary Carbonic Anhydrase I Immunoreactivity

BACKGROUND

Mild hemolysis occurs physiologically in neonates, but more severe forms can lead to life-threatening anemia. Newborns in developing regions are particularly at-risk due to the higher incidence of triggers (protozoan infections, sepsis, certain genetic traits). In advanced healthcare facilities, hemolysis is monitored indirectly using resource-intensive methods that probe downstream ramifications. These approaches could potentially delay critical decisions in early-life care, and are not suitable for point-of-care testing. Rapid and cost-effective testing could be based on detecting red blood cell (RBC)-specific proteins, such as carbonic anhydrase I (CAI), in accessible fluids (e.g., urine).

METHODS

Urine was collected from 26 full-term male neonates and analyzed for CAI using immunoassays (ELISA, western blot) and proteomics (mass spectrometry). The cohort included a range of hemolytic states, including admissions with infection, ABO incompatibility, and receiving phototherapy. Data were paired with hemoglobin, serum bilirubin (SBR), and C-reactive protein (CRP) measurements.

RESULTS

Urine from a control cohort (CRP < 20 mg/L, SBR < 125µmol/L) had no detectable CAI, in line with results from healthy adults. CAI excretion was elevated in neonates with raised SBR (>125 µmol/L), including those qualifying for phototherapy. Newborns with low SBR (<125 µmol/L) but elevated CRP (>20 mg/L) produced urine with strong CAI immunoreactivity. Proteomics showed that CAI was the most abundant RBC-specific protein in CAI-immunopositive samples, and did not associate with other RBC-derived peptides, indicating an intravascular hemolytic source followed by CAI-selective excretion.

CONCLUSIONS

CAI is a direct biomarker of intravascular hemolysis that can be measured routinely in urine using non-invasive methods under minimal-laboratory conditions.

Expression and immunolocalization of metallothioneins MT1, MT2 and MT3 in rat nephron

Rodent kidneys exhibit three isoforms of metallothioneins (MTs), MT1, MT2 and MT3, with poorly characterized localization along the nephron. Here we studied in adult male Wistar rats the renal expression of MTs mRNA by end-point RT-PCR and MT proteins by immunochemical methods The expression pattern of MT1 mRNA was cortex (CO)>outer stripe (OS)=inner stripe (IS)=inner medulla (IM), of MT2 mRNA was IM>CO>IS=OS, and of MT3 mRNA was IM>CO=OS=IM. MT1/2-antibody stained with heterogeneous intensity the cell cytoplasm and nuclei in proximal tubule (PT) and thin ascending limb, whereas MT3-antibody stained weakly the cell cytoplasm in various cortical tubules and strongly the nuclei in all nephron segments. However, the isolated nuclei exhibited an absence of MT1/2 and presence of MT3 protein. In MT1/2-positive PT cells, the intracellular staining appeared diffuse or bipolar, but the isolated brush-border, basolateral and endosomal membranes were devoid of MT1/2 proteins. In the lumen of some PT profiles, the heterogeneously sized MT1/2-rich vesicles were observed, with the limiting membrane positive for NHE3, but negative for V-ATPase, CAIV, and megalin, whereas their interior was positive for CAII and negative for cytoskeleton. They seem to be pinched off from the luminal membrane of MT1/2-rich cells, as also indicated by transmission electron microscopy. We conclude that in male rats, MTs are heterogeneously abundant in the cell cytoplasm and/or nuclei along the nephron. The MT1/2-rich vesicles in the tubule lumen may represent a source of urine MT and membranous material, whereas MT3 in nuclei may handle zink and locally-produced reactive oxygen species.

Localization of a protein inhibitor of carbonic anhydrase in pig tissues

The protein inhibitor of carbonic anhydrase (CA), pICA, was localized in pig tissues by an immunohistochemical technique, using rabbit antipICA IgG. Staining for pICA was found in liver sinusoids and kidney glomeruli, where phagocytic cells are located, i.e. Kupffer and mesangial cells, respectively. pICA was not found inside parenchymal cells, or in tissues from striated muscle, heart, eye or lung. It is concluded that the function of pICA is perhaps to bind the carbonic anhydrase isozymes CA I, II, and III, released from erythrocytes into the blood circulation by intravascular haemolysis. The complex of CA-pICA in plasma may then be transported to the reticuloendothelial system, for degradation and reclamation of CA-bound zinc. This would be similar to the fate of the haemoglobin-haptoglobin complex for the recycling of iron.

Renal handling and plasma elimination kinetics of carbonic anhydrase isoenzymes I, II and III in the rat

Using a radio-immunosorbent technique, the levels of the carbonic anhydrase (CA) isoenzymes CA I, II and III in plasma (1-3 micrograms ml-1), lymph (0.5-1.6 micrograms ml-1) and urine (0.03-0.06 micrograms ml-1), were determined in the rat. The renal clearances of CA I, II and III were 11 +/- 3, 42 +/- 11 and 35 +/- 4 nl min-1 (g kidney wet wt)-1 (n = 4-5), respectively. After a single i.v. injection of purified native or 125I-labelled isoenzymes, the elimination of CA I, and CA III from plasma followed a bi-exponential decline, with half-times of 7 and 9 min for the rapid phase and 112 min for the slow phase, respectively. Nephrectomy decreased the rapid phase and the build-up of catabolites. Therefore, the rapid phase of CA I and III elimination is probably explained by filtration of unbound isoenzyme at the glomeruli and subsequent degradation by the proximal tubules. The plasma elimination curve for CA II was different and followed a mono-exponential decline, with a half-time of 210 min both in normal and nephrectomized animals. This indicates that CA II is not filtered at the glomeruli. However, in acute renal failure, with leaking tubular cells, CA II was excreted into the urine. The slow elimination of the major part of the isoenzymes from plasma is explained by the binding of CA I, II and III to a plasma protein, immunochemically similar to transferrin, forming a macromolecular complex with a mol wt of 114 +/- 2 kDa.

Distal renal tubular acidosis: the value of urinary pH, PCO2 and NH4+ measurements

Distal renal tubular acidosis (dRTA) is not a single disease. The experimental forms of the syndrome are unsatisfactory as models of the naturally occurring disease, not least because they are seldom complicated by nephrocalcinosis, which is present in the majority of patients with spontaneous disease and contributes to the renal tubular defects found in the syndrome. Impairment of minimal urine pH, reduced urine carbon dioxide tension (PCO2) during passage of alkaline urine, and reduced urinary ammonium (NH4+) excretion, have all been advocated as essential criteria for the diagnosis of dRTA. Minimal urine pH, measured during metabolic acidosis, sulphate infusion, or after oral frusemide, is the yardstick against which other criteria should be assessed. A reduced urinary PCO2 is commonly found in dRTA but is not specific for the syndrome and may be accounted for by tubular defects other than those involving reduced distal hydrogen ion secretion. NH4+ excretion is reduced in most patients with renal acidosis whatever the nature of the underlying renal disease; this function is closely related to nephron mass, and is not specifically impaired in renal tubular disease.