A new polyclonal antibody to the human erythrocyte urea transporter UT-B detects a broad band between 45 and 65 kDa in human erythrocytes and between 37 and 51 kDa in rat erythrocytes. In human erythrocytes, the UT-B protein is the Kidd (Jk) antigen, and Jk(a+b+) erythrocytes express the 45- to 65-kDa band. However, in Jk null erythrocytes [Jk(a−b−)], only a faint band at 55 kDa is detected. In kidney medulla, a broad band between 41 and 54 kDa, as well as a larger band at 98 kDa, is detected. Human and rat kidney show UT-B staining in nonfenestrated endothelial cells in descending vasa recta. UT-B protein and mRNA are detected in rat brain, colon, heart, liver, lung, and testis. When kidney medulla or liver proteins are analyzed with the use of a native gel, only a single protein band is detected. UT-B protein is detected in cultured bovine endothelial cells. We conclude that UT-B protein is expressed in more rat tissues than previously reported, as well as in erythrocytes.

Physiological and molecular data demonstrate that urea transport in kidney and erythrocytes is regulated by specific urea transporter proteins. The urea transporter in the terminal inner medullary collecting duct permits very high rates of regulated transepithelial urea transport and results in the delivery of large amounts of urea into the deepest portions of the inner medulla, where it is needed to maintain a high interstitial osmolality for concentrating the urine maximally. The urea transporter in erythrocytes permits these cells to lose urea rapidly as they ascend through the ascending vasa recta, thereby preventing loss of urea from the medulla. Urea lost from the medulla would decrease concentrating ability by decreasing the efficiency of countercurrent exchange, as occurs in individuals who lack the Kidd antigen. The recent cloning of cDNAs for these two urea transporters has begun to yield new insights into the mechanisms underlying acute and long-term regulation of urea transport and should permit exciting new insights in the future. This review focuses on the physiological and biophysical evidence that established the concept of urea transporters, the subsequent cloning of cDNAs for urea transporters, and the recent integrative studies into the regulation of urea transport. We also propose a new systematic nomenclature and a new structural model for urea transporters.

Normally, lithium is not present in significant amounts in body fluids (Ͻ0.2 mEq/L). However, lithium salts have been used therapeutically for almost 150 years, beginning with its use for the treatment of gout (or uric acid diathesis) in the 1850s (1). Although gout was believed to include symptoms of mania and depression, it wasn't until the 1880s that John Aulde and Carl Lange observed that lithium could be used to treat symptoms associated with depression, independent of gout (1). However, the use of lithium became problematic and was discarded due to the serious toxicity associated with the widespread use of lithium in tonics, elixirs, and as a salt substitute (1).The modern era of lithium usage as a pharmacologic agent began with its "rediscovery" in 1950 by Cade and the clinical studies by Schou in the 1950s that established lithium as an effective treatment of manic-depressive illness (1). Lithium is now the drug of choice for treating bipolar affective disorders. It is successful in improving both the manic and depressive symptoms in 70 to 80% of patients (2). Lithium may also be used to treat alcoholism, schizoaffective disorders, and cluster headaches (3). Thus, lithium is an indispensable pharmaceutical component of modern psychiatric therapy.Unfortunately, lithium also has a narrow therapeutic index, with therapeutic levels between 0.6 and 1.5 mEq/L (Table 1) (2-4). The optimal steady-state concentration of lithium for maintenance treatment of bipolar disorders is generally considered to be 0.6 to 1.2 mEq/L, with slightly elevated steadystate concentrations (0.8 to 1.5 mEq/L) indicated for the acute management of manic episodes (5). Because toxicity can occur at levels Ͼ1.5 mEq/L, lithium levels must be carefully monitored and lithium dosage adjusted as necessary. This is especially true following changes in other medications that alter renal function, such as angiotensin-converting enzyme (ACE) inhibitors or nonsteroidal anti-inflammatory drugs (NSAID). Nephrologists require a thorough understanding of lithium since it is excreted by the kidney and its toxic side effects commonly affect renal function. In addition, the treatment of lithium intoxication usually requires consideration of the need for acute hemodialysis, a decision that should only be made by a nephrologist.

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Uracil-DNA glycosylase from rat liver mitochondria, an inner membrane protein, has been purified approximately 575,000-fold to apparent homogeneity. During purification two distinct activity peaks, designated form I and form II, were resolved by phosphocellulose chromatography. Form I constituted approximately 85% while form II was approximately 15% of the total activity; no interconversion between the forms was observed. The major form was purified as a basic protein with an isoelectric point of 10.3. This enzyme consists of a single polypeptide with an apparent Mr of 24,000 as determined by recovering glycosylase activity from a sodium dodecyl sulfate-polyacrylamide gel. A native Mr of 29,000 was determined by glycerol gradient sedimentation. The purified enzyme had no detectable exonuclease, apurinic/apyrimidinic endonuclease, DNA polymerase, or hydroxymethyluracil-DNA glycosylase activity. A 2-fold preference for single-stranded uracil-DNA over a duplex substrate was observed. The apparent Km for uracil residues in DNA was 1.1 microM, and the turnover number is about 1000 uracil residues released per minute. Both free uracil and apyrimidinic sites inhibited glycosylase activity with Ki values of approximately 600 microM and 1.2 microM, respectively. Other uracil analogues including 5-(hydroxymethyl)uracil, 5-fluorouracil, 5-aminouracil, 6-azauracil, and 2-thiouracil or analogues of apyrimidinic sites such as deoxyribose and deoxyribose 5'-phosphate did not inhibit activity. Both form I and form II had virtually identical kinetic properties, and the catalytic fingerprints (specificity for uracil residues located in a defined nucleotide sequence) obtained on a 152-nucleotide restriction fragment of M13mp2 uracil-DNA were almost identical. These properties differentiated the mitochondrial enzyme from that of the uracil-DNA glycosylase purified from nuclei of the same source.