A family of mammalian Na+-dependent L-ascorbic acid transporters.

Vitamin C (L-ascorbic acid) is essential for many enzymatic reactions, in which it serves to maintain prosthetic metal ions in their reduced forms (for example, Fe2+, Cu+), and for scavenging free radicals in order to protect tissues from oxidative damage. The facilitative sugar transporters of the GLUT type can transport the oxidized form of the vitamin, dehydroascorbic acid, but these transporters are unlikely to allow significant physiological amounts of vitamin C to be taken up in the presence of normal glucose concentrations, because the vitamin is present in plasma essentially only in its reduced form. Here we describe the isolation of two L-ascorbic acid transporters, SVCT1 and SVCT2, from rat complementary DNA libraries, as the first step in investigating the importance of L-ascorbic acid transport in regulating the supply and metabolism of vitamin C. We find that SVCT1 and SVCT2 each mediate concentrative, high-affinity L-ascorbic acid transport that is stereospecific and is driven by the Na+ electrochemical gradient. Despite their close sequence homology and similar functions, the two isoforms of the transporter are discretely distributed: SVCT1 is mainly confined to epithelial systems (intestine, kidney, liver), whereas SVCT2 serves a host of metabolically active cells and specialized tissues in the brain, eye and other organs.     

Renin in mouse but not in rat submandibular glands

Renin was found in the submandibular glands of male Quackenbush mice in concentrations higher than has been reported for any tissue of any strain or species. However, no renin-like activity could e detected in glands from male and female Wistar rats using either pH 5.8 or 7.4 for assay and a radioimmunoassay specific for renin's reaction product, angiotensin I. Rabbit submandibular glands contained renin.     

药物发现的先驱者格特鲁德B·埃利昂(1918～1999)

G·B·埃利昂（CertrudeBelleElion）是本世纪获得诺贝尔生理学与医学奖仅有6位女性中的一位。尽管这位合理药物设计的先驱完全应该为自己所取得的成就感到骄傲，但她一点不关心这类华丽词藻和周围气氛。她真正关心的是她认为会对疾病本身产生最有效治疗的最好技术。从她1983年退休到1999年2月21日81岁时突然去世为止，实际上她从未停止过工作，有不少人已认识到，她只是表面上退休而已。G·B·埃利昂1918年出生于纽约，她的父母来自立陶宛和俄罗斯。在10岁时她的祖父和母亲死于癌症，这是她记忆中印象最深刻的事件。1937年她从亨特女子…     

An equivalency condition of nonsingularity in nonlinear semidefinite programming

<正> In this paper,an equivalency condition of nonsingularity in nonlinear semidefinite programming,which can be viewed as a generalization of the equivalency condition of nonsingularity for linearsemidefinite programming,is established under certain conditions of convexity.     

Substantial increase of protein stability by multiple disulphide bonds

Disulphide bonds can significantly stabilize the native structures of proteins. The effect is presumed to be due mainly to a decrease in the configurational chain entropy of the unfolded polypeptide. In phage T4 lysozyme, a disulphide-free enzyme, engineered disulphide mutants that crosslink residues 3-97, 9-164 and 21-142 are significantly more stable than the wild-type protein. To investigate the effect of multiple-disulphide bonds on protein stability, mutants were constructed in which two or three stabilizing disulphide bridges were combined in the same protein. Reversible thermal denaturation shows that the increase in melting temperature resulting from the individual disulphide bonds is approximately additive. The triple-disulphide variant unfolds at a temperature 23.4 degrees C higher than wild-type lysozyme. The results demonstrate that a combination of disulphide bonds, each of which contributes to stability, can achieve substantial overall improvement in the stability of a protein.     

Thyrotropin-releasing hormone does not inhibit lysine vasopressin-induced growth hormone secretion in normal men

Summary In order to establish whether thyrotropin-releasing hormone (TRH) inhibits lysine-vasopressin (LVP)-induced growth hormone (GH) release, six normal men were tested with LVP alone or in combination with TRH. LVP strikingly increased serum GH levels; this response was not altered by TRH. These results indicated that in man TRH is not involved in the control of GH secretion in response to LVP.