No abstract

accounts for their increased urinary excretion in renal tubular disorders. Unfortunately, finding them in excess is not specific to renal tubular disease because all three are excreted in increased quantity whenever the endogenous creatinine clearance is reduced-to 25-30 ml/minute in the case of RBP and /i2M,2 and to less than 55 ml/minute in the case of a1m.3Increased excretion of N-acetyl-f,-D-glucosaminidase (NAG), an enzyme found in the lysosomes of proximal renal tubule cells, should be more specific for tubular pathology because its molecular mass is large enough to preclude passage through the normal glomerular basement membrane. However, increased excretion of NAG only reflects active tubular damage and it has also been reported in patients with glomerulonephritis.4 Increased albumin excretion usually reflects glomerular disease but when the quantities are small it may be due to impaired tubular reabsorption: thus some albuminuria is regularly found in patients with renal tubular disorders.5 In the course of searching for an improved protein marker of renal tubular disease we discovered that fl2-glycoprotein-1 (fl2Gl) is excreted in increased quantity by patients with primary renal tubular disorders, despite its relatively high molecular mass.6 To determine whether increased excretion of ,2G1 might be a more specific marker for tubular malfunction than existing tests we have now investigated its excretion by patients with primary glomerular pathology, obstructive nephropathy, and polycystic kidney disease, comparing its excretion with that of albumin, RBP, a,M, fl2M, and NAG. The study was approved by the hospital ethical committee.

Urinary ,32-glycoprotein-1 was measured in 60 patients with conditions recognised as causing renal tubular impairment and compared with established markers of early tubular malfunction. Increased /32-glycoprotein-1 excretion was found in 49 (82%) of the subjects; raised excretion of a,-microglobulin, retinol-binding protein, and fi2-microglobulin was found in 46 (77%), 45 (75%), and 31 (52%), respectively, and increased urinary Nacetyl-j3-D-glucosaminidase activity in 32 of 54 of the subjects (59%). The increase was particularly pronounced in those with proximal tubule malfunction, although considerable variation occurred.f2-glycoprotein-1 was shown to be stable in urine over the physiological pH range, and it is concluded that its measurement provides a means of detecting chronic malfunction of the renal tubules that is marginally more sensitive than assays of cl-microglobulin or retinol-binding protein, and more reliable than assays of f2-microglobulin or N-acetyl-f,-D-glucosaminidase.

A surface-associated sulphydryl (thiol) protein (SASP) constitutively present in most nucleated cells was purified from human THP-1 monocytes and rat C6 glioma cells. The human protein was similar in mass and isoelectric point and had the same N-terminal amino acid sequence to adult T-cell leukemia-derived factor (ADF), a growth factor secreted by human lymphoid cells which is able to induce increased expression of interleukin-2 receptors. A further internal amino acid sequence, determined following cleavage of human SASP with cyanogen bromide, was also identical to the corresponding sequence deduced for ADF. Samples of SASP were able to reductively depolymerize human immunoglobulin, a property shared with thioredoxin, a ubiquitous protein, almost identical to ADF, with an essential function in many thiol-dependent reducing reactions. Furthermore, SASP purified from rat C6 glioma cells had an identical N-terminal amino acid sequence to that deduced for rat liver thioredoxin, showing that they were both members of the same family of proteins. The use of membrane-impermeable thiol reagents indicated that SASP was predominantly a cell-surface protein, and was not normally secreted. This SASP protein appeared to be a surface-associated form of thioredoxin that was constitutively present in a wide range of cells and was related to ADF, a secreted form of the same protein.

SUMMARY.A solid-phase sandwich enzyme-linked immunosorbent assay for determining 0,-glycoprotein I in urine has been developed. It has a working concentration range of 5-40 pg/L and a detection limit of approximately 1 . 4 pg/L. The withinplate coefficient of variation (CV) falls between 1.4% and 2 -1 % , and the betweenbatch CV ranges from 5.2 to 6.0%. Recovery of P,-glycoprotein I added to urine varies between 96 and 110%. The assay can also be used for determining P,-glycoprotein I in serum. Additional key phrases: Apolipoprotein-H; renal tubular disordersBeta-2-glycoprotein I was identified in the perchloric acid soluble fraction of human plasma in 1961.' It has a molecular mass of approximately 50 kDa and has been shown to consist of 326 amino acids with five attached glucosaminecontaining oligosaccharides.2 Several forms have been found and genetic studies indicate that three allelic variants are coded at a single gene IOCUS.~