Plasma membrane-associated increase in guanylate cyclase activity in regenerating rat liver

Rapid purification of dipeptidyl peptidase IV from rat liver plasma membrane

Dipeptidyl peptidase IV (EC 3.4.14.5) was solubilized from rat liver plasma membranes with sulphobetaine 14 and purified by successive affinity chromatography on Con A-Sepharose, wheat germ lectin-Sepharose and arginine-Sepharose columns. The specific activity of the final preparation was 49.4 mumol Gly-Pro p-nitroanilide/min per mg protein, representing a 1098-fold purification of the homogenate. SDS-polyacrylamide gel electrophoresis of the arginine-Sepharose eluate showed a single protein band with a molecular weight of 105,000. The isoelectric point was determined to be 3.9 under non-denaturing conditions with sulphobetaine 14. The preparation was free of post-proline cleaving enzyme. The content of aminopeptidase M was 0.2% of the total protein.

Cyclic nucleotides in cerebrospinal fluid of patients with intracranial and spinal tumors

Since the cyclic nucleotides (CN) adenosine 3',5'-monophosphate (cAMP) and guanoside 3',5'-monophosphate (cGMP) are involved in the regulation of cell proliferation and tumor growth in vitro, a study was made of the levels of these compounds in the lumbar cerebrospinal fluid (CSF) of 20 patients with intracranial and spinal tumors. In the presence of benign or malignant intracranial tumors there was a slight and not significant decrease of cAMP as well as cGMP levels in the CSF, as compared to control patients. While there was no significant correlation between the levels of the two CN in controls, there was a positive correlation in tumor patients. Total protein content and cAMP were negatively correlated in malignant intracranial tumors. Possible influences of tumor growth and intracranial pressure increases on CN levels are discussed. In spinal tumor patients normal CN levels were observed. However, in a patient with meningeal sarcoma an extremely marked elevation of cAMP occurred in parallel with the extension of the tumor to the spinal meningeal space, suggesting massive secretion of cAMP from the tumor cells.

Production and characterization of monoclonal antibodies to soluble rat lung guanylate cyclase

Four monoclonal antibodies to rat lung soluble guanylate cyclase [GTP pyrophosphate-lyase (cyclizing) EC 4.6.1.2] have been produced by fusing spleen cells from immunized BALB/c mice with SP-2/0 myeloma cells. The antibodies were detected by their ability to bind immobilized guanylate cyclase and by immunoprecipitation of purified enzyme in the presence of second (rabbit anti-mouse) antibody. After subcloning by limiting dilution, hybridomas were injected intraperitoneally into mice to produce ascitic fluid containing 2-5 mg of antibody per ml. The four antibodies obtained had titers of between 1:1580 and 1:3160 but were detectable at dilutions greater than 1:20,000. Soluble guanylate cyclase from several rat tissues were crossreactive with the four monoclonal antibodies, suggesting that the soluble enzyme from different rat tissues is antigenically similar. The antibodies also recognized soluble lung enzyme from rat, beef, and pig, while enzyme from rabbit was not crossreactive and mouse enzyme was recognized by only one of the antibodies. Particulate guanylate cyclase from a number of tissues had only minimal crossreactivity with the antibodies. Immunoprecipitated guanylate cyclase retained catalytic activity, could be activated with sodium nitroprusside, and was inhibited by cystamine. None of the antibodies were inhibitory under the conditions examined. These antibodies will be useful probes for the study of guanylate cyclase regulation and function under a variety of physiological conditions.

Characterization of rat testicular guanylate cyclase during development

The biochemical characteristics of rat testicular guanylate cyclase were investigated and the activity and subcellular distribution of the enzyme was determined during testicular development. Examination of the effects of metal ions, nucleotides, detergents and other in vitro activators on the activity of guanylate cyclase revealed that the testicular enzyme is similar in most respects to guanylate cyclase isolated from other mammalian tissues. Changes in the total activity of guanylate cyclase during testicular development paralleled changes in the tissue concentration of cyclic GMP; i.e. guanylate cyclase activity and tissue cyclic GMP were highest during the early stages of development. Subcellular fractionation revealed that the activity of the soluble form of guanylate cyclase was best correlated with tissue cyclic GMP. Biochemical analysis of the soluble enzyme prepared from testes of neonatal and adult rats did not reveal any significant differences in the characteristics of the enzyme during ontogeny with the exception of a 2.5 fold increase in V noted in the neonatal testis. The results of this study are consistent with a molecular mechanism that allows independent regulation of the different forms of guanylate cyclase.

Cytidine 3',5'-monophosphate phosphodiesterase: decreased activity in the regenerating and developing liver

A decrease in the activity of the enzyme cytidine 3',5'-monophosphate (cyclic CMP) phosphodiesterase was noted in the regenerating liver of young rats as early as 8 hours after partial hepatectomy, with a maximum decrease occurring 12 hours after the surgery. In comparison, in old rats which showed a slower liver growth, the maximum decrease in the activity of cyclic CMP phosphodiesterase was smaller and occurred at a much later time (2 days after surgery). A similar decrease in the enzyme activity was observed in the fetal liver of guinea pigs. These findings suggest that regulation of tissue concentration of cyclic CMP may be crucial for the regeneration and development of the liver.

Induction and 'superinduction' of sialylation of membrane-bound gamma-glutamyltransferase during liver regeneration

The present paper shows that in the regenerating rat liver the membrane-bound-gamma-glutamyltransferase exists in two molecular forms. Depending on the state of proliferation, a sialic-acid-rich enzyme (in the fetal or regenerating liver) or a sialic-acid-poor enzyme (in the adult or quiescent liver) could be detected. In regeneration liver (24 h after 2/3 resection) only the sialic-acid-rich or fetal enzyme could be found. Since total enzyme activity (adult + fetal type) remained unchanged, it is assumed that the adult type of gamma-glutamyltransferase was modified by sialylation during the initial phase of liver regeneration. This process of sialylation was prevented by inhibitors of RNA or protein synthesis such as D-galactosamine, actinomycin D or cycloheximide, provided that the inhibitor (D-galactosamine) was given within the first 8 h after partial hepatectomy. Sialylation was not impaired by inhibitors of DNA synthesis, e.g. hydroxyurea or cytosine arabinoside. Administration of actinomycin D during a defined phase of proliferation (24 to 48 h after partial hepatectomy) stimulated the transfer of sialic acid to gamma-glutamyltransferase, a finding which describes for the first time the so-called 'superinduction' of a sialylation process.

Protein-catabolic stage of isolated rat hepatocytes

Isolated rat hepatocytes in suspension are in a protein-catabolic state (negative nitrogen balance), as measured by the continuous release of nitrogen in the form of amino acids and urea. The nitrogen loss corresponds to a protein degradation rate of 3-4% per h, while the rate of protein synthesis is negligible. Cells prepared from fasted, fed to regenerating livers are all highly protein-catabolic. The nitrogen balance is unaffected by insulin or amino acids (physiological mixture), and various metabolites and sera have only moderate effects. However, incubation of the cells for 2-4 h in a tissue culture medium (Dulbecco's) reduces the nitrogen loss dramatically, suggesting the formation of an anticatabolic factor under these conditions.