Intramolecular disulfide bonds of the prolactin receptor short form are required for its inhibitory action on the function of the long form of the receptor.

The short form (S1b) of the prolactin receptor (PRLR) silences prolactin-induced activation of gene transcription by the PRLR long form (LF). The functional and structural contributions of two intramolecular disulfide (S-S) bonds within the extracellular subdomain 1 (D1) of S1b to its inhibitory function on the LF were investigated. Mutagenesis of the paired cysteines eliminated the inhibitory action of S1b. The expression of the mutated S1b (S1bx) on the cell surface was not affected, indicating native-like folding of the receptor. The constitutive JAK2 phosphorylation observed in S1b was not present in cells expressing S1bx, and JAK2 association was disrupted. BRET(50) (BRET(50) represents the relative affinity as acceptor/donor ratio required to reach half-maximal BRET [bioluminescence resonance energy transfer] values) showed decreased LF/S1bx heterodimeric-association and increased affinity in S1bx homodimerization, thus favoring LF homodimerization and prolactin-induced signaling. Computer modeling based on the PRLR crystal structure showed that minor changes in the tertiary structure of D1 upon S-S bond disruption propagated to the quaternary structure of the homodimer, affecting the dimerization interface. These changes explain the higher homodimerization affinity of S1bx and provide a structural basis for its lack of inhibitory function. The PRLR conformation as stabilized by S-S bonds is required for the inhibitory action of S1b on prolactin-induced LF-mediated function and JAK2 association.

Hypotensive, hypoglycaemic and toxicological studies on the flavonol C-glycoside shamimin from Bombax ceiba.

Shamimin, a C-flavonol glucoside from Bombax ceiba leaves showed significant potency as a hypotensive agent at the doses of 15 mg/kg, 3 mg/kg, 1 mg/kg and significant hypoglycaemic activity at 500 mg/kg in Sprague-Dawley rats. Further studies revealed that it did not cause any mortality in mice at the dose of 1 g/kg but in rats 500 mg/kg is a lethal dose. Aqueous and methanolic extracts of Bombax ceiba leaves and one of its fractions were also subjected to pharmacological and toxicological screening.

Carbohydrate moiety of Plasmodium falciparum glycoproteins: the nature of the carbohydrate-peptide linkage in the MSP-2 glycoprotein.

Metabolic labelling of Plasmodium falciparum parasites with [3H]GlcN, [3H]Man, [3H]Gal and [3H]ethanolamine, and subsequent purification by SDS-PAGE of the labelled material provided effective labelling of the MSP-1, 195 kDa, and MSP-2, 42-53 kDa, glycoproteins. Reductive beta-elimination of the MSP-2 released from the gel consisted of glycopeptides containing labelled sugars. Processing of the eliminated components and identification of the sugar residues demonstrated the presence of N-acetylglucosaminitol and N-acetylgalactosaminitol amongst other labelled sugars. Reductive beta-elimination with sodium hydroxide-sodium borotritide-borohydride showed the presence of glucosaminitol and alanine in the hydrolysis products. The MSP-2 was retained on solid phase wheat-germ agglutinin and was released from the lectin by treatment with GlcNAc. Upon treatment with O-glycanase the MSP-2 glycoprotein released labelled amino sugar, and derived oligosaccharides on treatment with exoglycosidases released labelled components corresponding to the metabolically incorporated sugars. Labelled Gal was incorporated into the MSP-2 glycoprotein using [3H]UDP-Gal and galactosyltransferase. The galactosylated glycoprotein released labelled Gal upon treatment with beta-galactosidase. The results of the present study suggest that the carbohydrate chains of the MSP-2 glycoprotein are attached to the protein backbone via GlcNAc- and GalNAc-serine/threonine in O-glycosyl linkage and the glycoprotein has terminal GlcNAc and Gal residues. The carbohydrate moieties of MSP-2, glycoprotein consist mainly of short chains linked to the protein core.