The role of brush border enzymes in tubular reabsorption of disaccharides: a microperfusion study in rat kidney

Pseudohyponatremia: Mechanism, Diagnosis, Clinical Associations and Management

Pseudohyponatremia remains a problem for clinical laboratories. In this study, we analyzed the mechanisms, diagnosis, clinical consequences, and conditions associated with pseudohyponatremia, and future developments for its elimination. The two methods involved assess the serum sodium concentration ([Na]_S) using sodium ion-specific electrodes: (a) a direct ion-specific electrode (ISE), and (b) an indirect ISE. A direct ISE does not require dilution of a sample prior to its measurement, whereas an indirect ISE needs pre-measurement sample dilution. [Na]_S measurements using an indirect ISE are influenced by abnormal concentrations of serum proteins or lipids. Pseudohyponatremia occurs when the [Na]_S is measured with an indirect ISE and the serum solid content concentrations are elevated, resulting in reciprocal depressions in serum water and [Na]_S values. Pseudonormonatremia or pseudohypernatremia are encountered in hypoproteinemic patients who have a decreased plasma solids content. Three mechanisms are responsible for pseudohyponatremia: (a) a reduction in the [Na]_S due to lower serum water and sodium concentrations, the electrolyte exclusion effect; (b) an increase in the measured sample's water concentration post-dilution to a greater extent when compared to normal serum, lowering the [Na] in this sample; (c) when serum hyperviscosity reduces serum delivery to the device that apportions serum and diluent. Patients with pseudohyponatremia and a normal [Na]_S do not develop water movement across cell membranes and clinical manifestations of hypotonic hyponatremia. Pseudohyponatremia does not require treatment to address the [Na]_S, making any inadvertent correction treatment potentially detrimental.

Osmotic Nephropathy Resulting from Maltose-Based Intravenous Immunoglobulin Therapy

Intravenous immunoglobulin preparations are being used for an increasing number of indications. To minimize adverse reactions, sugar additives such as sucrose, maltose, and glycine are added to some preparations to serve as stabilizing agents. Intravenous immunoglobulin infusion induces acute renal failure (ARF) via a mechanism of osmotic nephrosis. Most reported cases are related to the use of sucrose-based intravenous immunoglobulin. Herein, we describe a patient with lupus nephritis treated with an immunoglobulin preparation containing maltose who developed ARF with histologic changes characterized by vacuolization and swelling of renal proximal tubular cells. Our case draws nephrologists' attention to the potential of maltose-based immunoglobulin in producing renal failure. Awareness and exercising caution in high-risk groups is elementary to the prevention of this condition.

Serum angiotensin converting enzyme levels in patients with alpha 1-antitrypsin variants

Serum angiotensin converting enzyme levels were measured in 184 subjects having either MM, MZ, ZZ, or MS Pi-types of alpha 1-antitrypsin. Elevated angiotensin converting enzyme levels were detected in 31 percent of the patients with the MZ Pi-type, 20 percent of the patients with the ZZ Pi-type, and 20 percent of the patients with the MS Pi-type compared with 1.33 percent of those with normal MM Pi-type. The mean serum angiotensin converting enzyme levels were also significantly higher in those with the MZ, ZZ, and MS Pi-types. Multiple family members of two families were found to have both the Z variant and angiotensin converting enzyme elevations, suggesting the possibility of a genetic linkage. Alpha 1-antitrypsin deficiency must be added to the list of disease states potentially associated with elevated serum angiotensin converting enzyme levels.

Purification by affinity chromatography and characterization of a neutral alpha-glucosidase from horse kidney

A horse kidney neutral alpha-D-glucosidase (alpha-D-glucoside glucohydrolase, EC 3.2.1.20) was purified about 580-fold with a yield of 33% by an affinity chromatography technique using the p-aminophenyl-beta-D-maltoside, a substrate derivative, as ligand. The purified enzyme, homogeneous in polyacrylamide gel electrophoresis, was a glycoprotein with a molecular weight of 280 000 as calculated by gel filtration and its isoelectric focusing points was found to be pH 4.1. The purified enzyme was able to hydrolyze various substrates having (alpha-1,2), (alpha-1,3), (alpha-1,4), and (alpha-1,6) glucosidic linkages. The V/Km ratio shows that the (alpha-1,4) linkages are the best substrates. The pKm of the purified enzyme determined at different pH values indicated that two ionizable groups with pK values 5.2 and 6.9 could be essential in the active site. Enzyme modification with cardodiimide abolished the maltase activity. The turanose, a substrate analogue, protected the enzyme against this inactivation.

Quantitative distribution of lysosomal hydrolases in the rat nephron

The activities of N-acetyl-beta, D-glucosaminidase (NAG, EC 3.2.1.30), beta, D-galactosidase (beta-gal, EC 3.2.1.23) and acid phosphatase (ac-Pase, EC 3.1.3.2) were measured in the glomeruli, five segments of the proximal and four segments of the distal tubule of normal male Wistar rats. The activities of NAG and beta-gal are 3- to 5-fold higher in the first part of the proximal tubule than in other segments and very low in glomeruli. We propose that the distribution of these two glycosidases reflects the contribution of the different tubular segments to the reabsorption of glycoproteins. The maximal activity of ac-Pase was found in the straight part of the proximal tubule. It was only 1.5-fold higher than in the distal tubule. Moreover, the activity in glomeruli is rather high. We conclude that ac-Pase is not primarily involved in the handling of reabsorbed molecules.

The role of brush border enzymes in tubular reabsorption of disaccharides: a microperfusion study in rat kidney

Renal tubular reabsorption of maltose, sucrose and lactose were studied in vivo et situ by continuous microperfusion of single proximal convolutions of rat kidney. The 14C-label of maltose (2.5 mmol/l) was removed from the lumen of the proximal tubule at about the same rate as found for glucose. Maltose reabsorption was completely inhibited in presence of 30 mmol/l glucose or of 0.1 mmol/l phlorizin. Chemical analysis of the samples showed a complete conversion of maltose into glucose within a perfusion distance of 2 mm. It is concluded from these results that within the tubular lumen maltose is split very rapidly by a brush border glucosidase. The short half time of this process permits the breakdown product glucose to be almost completely reabsorbed subsequently within the proximal tubule. In contrast, sucrose and lactose were neither split nor reabsorbed by the tubule brush border.