The inhibition of nucleic acid synthesis by mycophenolic acid

1. Mycophenolic acid, an antibiotic of some antiquity that more recently has been found to have marked activity against a range of tumours in mice and rats, strongly inhibits DNA synthesis in the L strain of fibroblasts in vitro. 2. The extent of the inhibition of DNA synthesis is markedly increased by preincubation of the cells with mycophenolic acid before the addition of [(14)C]thymidine. 3. The inhibition of DNA synthesis by mycophenolic acid in L cells in vitro is reversed by guanine in a non-competitive manner, but not by hypoxanthine, xanthine or adenine. 4. The reversal of inhibition by guanine can be suppressed by hypoxanthine, 6-mercaptopurine and adenine. 5. Mycophenolic acid does not inhibit the incorporation of [(14)C]thymidine into DNA in suspensions of Landschütz and Yoshida ascites cells in vitro. 6. Mycophenolic acid inhibits the conversion of [(14)C]hypoxanthine into cold-acid-soluble and -insoluble guanine nucleotides in Landschütz and Yoshida ascites cells and also in L cells in vitro. There is some increase in the radioactivity of the adenine fraction in the presence of the antibiotic. 7. Mycophenolic acid inhibits the conversion of [(14)C]hypoxanthine into xanthine and guanine fractions in a cell-free system from Landschütz cells capable of converting hypoxanthine into IMP, XMP and GMP. 8. Preparations of IMP dehydrogenase from Landschütz ascites cells, calf thymus and LS cells are strongly inhibited by mycophenolic acid. The inhibition showed mixed type kinetics with K(i) values of between 3.03x10(-8) and 4.5x10(-8)m. 9. Evidence was also obtained for a partial, possibly indirect, inhibition by mycophenolic acid of an early stage of biosynthesis of purine nucleotides as indicated by a decrease in the accumulation of formylglycine amide ribonucleotide induced by the antibiotic azaserine in suspensions of Landschütz and Yoshida ascites cells and L cells in vitro.     

Cytological aspects of haemoglobin and chlorocruorin synthesis in polychaete annelids

Summary The ultrastructure of the haematopoietic cells in the polychaetes Neoamphitrite figulus Dalyell, Lanice conchilega (Pallas), Arenicola marina (L.), Myxicola infundibulum Renier, Megalomma vesiculusom (Montagu), Sabella penicillus L., are compared: all show similarities in having well developed Golgi, granular endoplasmic reticulum and haemoglobin or chlorocruorin in vesicles, and numerous mitochondria. The porphyrin byproducts of synthesis are combined with iron as haematins within electrondense granules built up from multi-lamellar organelles. The structure of the basal lamina which alone separates the cells from the lumen of the vessels is described and evidence is presented for the method of release of the haem into the plasma by reverse pinocytosis. The cycle of synthesis within the cell is discussed and the process of haem synthesis in annelids is reviewed. The structure of the haemoglobin-containing coelomocytes of Neoamphitrite figulus is briefly described.This work was made possible by an award from the Science Research Council. We would also like to thank the Director and Staff of the Plymouth Laboratory, where some of the work was done, for their hospitality and facilities.     

Metabolism of larger high density lipoproteins accumulating in some families of baboons fed a high cholesterol and high saturated fat diet

Progeny of certain baboon sires accumulate lipoproteins in high density lipoprotein-1 (HDL1) when challenged with a high cholesterol, high saturated fat diet. These studies were conducted to determine the apoprotein composition and metabolic fate of HDL1 in the plasma. HDL1 particles containing apoA-I with and without apoE were detected. The majority of particles, however, contained apoA-I without any detectable apoE. To determine the metabolic fate of HDL1 in plasma, HDL1 labeled with iodinated apoA-I from animals with high levels of HDL1 and iodinated apoA-I-labeled autologous HDL were coinjected into both high and low HDL1 animals. The data for the decay of radioactivity in HDL1 and HDL were analyzed by multicompartment modelling. The radioactivity from HDL1 was cleared from the plasma either via direct removal (9.1 +/- 4.7% in low and 21.7 +/- 8.3% in high HDL1 animals) or via its conversion to HDL. A large proportion of radioactivity from HDL1 was rapidly transferred to HDL directly or metabolized via an intermediate compartment. Most of the radioactivity from apoE-poor HDL1, however, was transferred to HDL. Both high and low HDL1 animals catabolized HDL1 and HDL similarly. Low HDL1 animals transferred HDL1 radioactivity to HDL much faster. No detectable radioactivity from HDL was transferred to HDL1. Thus, HDL1 that accumulates in high HDL1 animals is mainly a precursor for HDL. Our hypothesis is that this accumulation of HDL1 is due to the slower cholesteryl ester transfer from HDL to lower density lipoproteins, thus affecting reverse cholesterol transport in high HDL1 baboons.     

The effects of a glycerin-based blood analog on the testing of bioprosthetic heart valves

Detailed comparisons of aortic valvular flow using saline, with that using a glycerin-based blood analog in a pulse duplicator are reported. The experiments were carried out to determine whether exposure to glycerin caused stiffening of bioprosthetic valve leaflets. For two pericardial bioprostheses and for a mechanical valve we observed a fluid-dependent systolic volume flow, a fluid-dependent regurgitation volume, and fluid-dependent systolic pressure differences. Volume flow changes, both forward and reverse, are independent of valve type. The observed pressure differences, while proportional to fluid density for the mechanical valve, are fluid dependent in a more complicated way for the pericardial valves. However, no trend of changing valvular performance was observed over as much as 80 days of glycerin exposure, indicating that it is unlikely that the fluid-dependent performance was caused by glycerin absorption by the valve leaflets. We conclude that valid performance comparisons between mechanical and bioprosthetic valves may be made using a glycerin-based fluid. Furthermore, it appears that any detailed analysis of the physical mechanisms of valvular flow dissipation will require a properly matched blood analog.     

Tonic activity of submucosal neurons influences basal ion transport

The influence of tonically active submucosal neurons on basal ion transport was studied using sheets of guinea pig ileum set up in flux chambers. Tetrodotoxin evoked an immediate and sustained decrease in short-circuit current that was sustained for 60 minutes compared with control tissues in which basal currents gradually decreased over time. Time-dependent changes in basal short-circuit currents in tissues treated with atropine were not significantly different from control tissues. The decrease in short-circuit current after tetrodotoxin resulted from a greater increase in net chloride absorption than sodium absorption. Changes in net sodium and chloride transport were due to an increase in the mucosal-to-serosal fluxes of these ions. The results suggest that tonic activity of submucosal neurons limits the absorptive capacity of the guinea pig ileum.     

Molecular genetic study of the frequency of monosomy 22q11 in DiGeorge syndrome

It is well established that DiGeorge syndrome (DGS) may be associated with monosomy of 22q11-pter. More recently, DNA probes have been used to detect hemizygosity for this region in patients with no visible karyotypic abnormality. However, DGS has also been described in cases where the cytogenetic abnormality does not involve 22q11; for instance, four cases of 10p- have been reported. In this study we have prospectively analyzed patients, by using DNA markers from 22q11, to assess the frequency of 22q11 rearrangements in DGS. Twenty-one of 22 cases had demonstrable hemizygosity for 22q11. Cytogenetic analysis had identified interstitial deletion in 6 of 16 cases tested; in 6 other cases no karyotype was available. When these results are combined with those from our previous studies, 33 of 35 DGS patients had chromosome 22q11 deletions detectable by DNA probes.