Transfer of N, N'-diacetylchitobiose from dolichyl diphosphate into a gray matter membrane glycoprotein

Gray matter and white matter membranes catalyze the transfer of label from UDP-N-acetyl-[${}^{14}\text{C}$] glucosamine into N-acetyl[${}^{14}\text{C}$]glucosaminyl-pyrophosphoryl-dolichol, N,N′-diacetyl [${}^{14}\text{C}$]chitobiosyl-pyrophosphoryl-dolichol, and N-acetyl[${}^{14}\text{C}$]glucosamine-labeled glycoprotein. Gel filtration of the Pronase digests of gray matter N-acetyl[${}^{14}\text{C}$]glucosamine-labeled glycoprotein reveals two N-acetyl[${}^{14}\text{C}$]glucosamine-labeled glycopeptide fractions. One fraction (A) contains approximately eight glycose units. All of the radioactivity is at nonreducing termini and can be released by treatment with an exo-β-N-acetylglucosaminidase. A smaller N-acetyl[${}^{14}\text{C}$]glucosamine-labeled glycopeptide (B) is recovered in the elution volume expected for an asparaginyl disaccharide. Structural studies show that the labeled saccharide unit in glycopeptide B is N,N′-diacetyl[${}^{14}\text{C}$]chitobiose. The linkage between the ${}^{14}\text{C}$-labeled disaccharide and the polypeptide has the properties of an N-glycosidic attachment to asparagine. Only the larger N-acetyl[${}^{14}\text{C}$]glucosamine-labeled glycopeptide (A) is found in Pronase digests of white matter membrane N-acetyl[${}^{14}\text{C}$]glucosamine-labeled glycoprotein after incubation with UDP-N-acetyl[${}^{14}\text{C}$]glucosamine. When gray matter membranes are incubated with UDP-N-acetyl[${}^{14}\text{C}$]glucosamine in the presence of tunicamycin or UMP, the labeling of glycolipid and the asparaginyl disaccharide is inhibited. UMP and tunicamycin have no effect on the transfer of N-acetyl[${}^{14}\text{C}$]glucosamine to external acceptor sites of the larger glycopeptide (A). The transfer of N,N′-diacetyl[${}^{14}\text{C}$]-chitobiose from carrier lipid to protein is observed when extensively washed membranes containing endogenous, prelabeled ${}^{14}\text{C}$-labeled glycolipids are incubated in the presence or absence of unlabeled GDP-mannose. UMP treatment of the prelabeled membranes selectively discharged over 80% of the label from N-acetyl[${}^{14}\text{C}$]glucosaminyl-pyrophosphoryl-dolichol, but had no effect on the transfer of the ${}^{14}\text{C}$-labeled disaccharide to protein. All of these results are concordant with transfer of N,N′-diacetylchitobiose from dolichyl diphosphate to gray matter glycoprotein. The major membrane glycoprotein labeled by the lipid-mediated [${}^{14}\text{C}$]disaccharide transfer reaction has an apparent molecular weight of 24,000. Tunicamycin prevents the enzymatic labeling of the gray matter glycoprotein having an apparent molecular weight of 24,000.     

The inhibition of phospholipase A2 by manoalide and manoalide analogues

Manoalide, a natural product from sponge, displays anti-inflammatory activity in vivo. Previous work has shown that manoalide is also a potent covalent inhibitor of the extracellular phospholipase A2 from cobra venom and that the inhibition correlated with a pH-dependent change in manoalide (Lombardo and Dennis (1985) J. Biol. Chem. 260, 7234-7240). Manoalide contains two rings and the opening of either would produce an alpha,beta-unsaturated aldehyde. The cobra venom phospholipase A2 may be able to catalyze the opening or isomerization of one of these rings, raising the possibility that manoalide is acting as a suicide substrate. To ascertain the role of the gamma-lactone ring in the inhibition, we have now investigated a synthetic manoalide analogue, 3(cis,cis-7,10)-hexadecadienyl-4-hydroxy-2-butenolide (HDHB) which contains only the alpha,beta-unsaturated gamma-lactone ring. We have found that the closed and open forms are in rapid equilibrium between pH 4 and 9 with the cyclic form being preferred at acidic pH values and the open cis form preferred at pH 9.5. When the pH is raised above 12, the alpha,beta double bond isomerizes to form trans-HDHB. Once the trans compound is formed, it is stable at all pH values and does not recyclize to the gamma-lactone ring. The observed pKa of 7.7 found for the inhibition of manoalide agrees well with the transition of the closed to the cis form of the gamma-lactone ring. Kinetic experiments with the HDHB compound show that under conditions in which the cis and closed form of the inhibitor are present in equal molar ratios, HDHB is not an irreversible inhibitor, but reversibly competes with substrate. However, the kinetics of this inhibition are complex and do not follow either pure competitive or non-competitive inhibition. The trans-HDHB exhibits similar complex kinetic but is several times more potent. The distinct differences between the behavior of manoalide and HDHB clearly indicate that while the gamma-lactone ring may play an important role in manoalide inhibition, it alone does not produce irreversible inhibition.     

Immunological characterization of cytochrome H-450 in rat tissues

Rabbit antiserum produced against rat liver cytochrome H-450 was specific for cytochrome H-450. The antiserum did not react with hemolysate, microsomal and mitochondrial fractions of liver, and tissue extracts from heart, lung skeletal muscle, and testis of rat. With the monospecific antiserum, a rocket immunoelectrophoretic assay method was developed for the quantitation of the antigen with a sensitivity of 25 ng. By using rocket immunoelectrophoresis, the total amounts of the antigen found in liver, kidney, and brain of 20 rats were 33.6, 3.6, and 1.3 mg, respectively. It appears that the antigens in liver, kidney, and brain are immunologically identical. From immunological studies with subcellular fractions of rat liver, the antigen was found only in the postmicrosomal fraction. This indicates that the antigen is not a precursor or a proteolytic product of known cytochromes in mitochondria or microsomes. Therefore, cytochrome H-450 is a unique cytosolic protein found in brain, kidney, and liver.     

Immunochemical cross-reactivity between cyanogen bromide fragments of human serum albumin

The antigenic structure of human albumin was investigated in order to establish whether or not there was any similarity between its antigenic sites. Using immunoadsorbent columns prepared with cyanogen bromide fragments of human serum albumin, antibodies directed against different portions of the albumin molecules were isolated. Measurement of the amount of the antibodies isolated and study of their specificity by inhibition techniques show that these subpopulations of antibodies reacted not only with the fragment used for their isolation (homologous) but also with the other fragments (heterologous). Heterologous fragments were inhibiting only at a very high concentration with regard to the homologous ones. These results show that there is a weak cross-reactivity between different portions of the albumin molecule. This reaction is most probably due to the homology existing in the sequence of the human albumin molecule which has arisen by gene duplication. The same type of behavior can be predicted to extent to other molecules which have evolved by similar mechanisms.