Identity of a cytosolic neutral cholesterol esterase in rat liver with the bile salt stimulated cholesterol esterase in pancreas

A neutral cholesterol esterase has been purified to homogeneity from the cytosolic fraction of rat liver. The 105,000 x g supernatant fraction of rat liver was applied to a DEAE-cellulose column to isolate a partially purified fraction of hepatic cholesterol esterase. Immunoblot analysis of the partially purified liver fraction with the anti-porcine pancreatic cholesterol esterase IgG demonstrated a single band with a molecular weight of 67,000. The hepatic protein was then isolated by immunoaffinity chromatography technique using a column constructed with antibodies prepared against the pancreatic cholesterol esterase. Characterization of the hepatic cholesterol esterase revealed that the hepatic enzyme shared antigenic epitopes with the pancreatic cholesterol esterase and was similarly activated by addition of bile salt such as taurocholate. Moreover, amino-terminal sequencing analysis of the hepatic cholesterol esterase showed an identical sequence with the pancreatic enzyme. Taken together, these results showed that the cholesterol esterases in the liver and the pancreas are very similar and possibly identical proteins.     

Specific-locus mutations induced in eukaryotes (especially mammalian cells) by radiation and chemicals: a perspective

In the course of discovering the first mutagen (X-rays) just over 60 years ago, Herman J. Muller asked whether X-rays induced single-gene mutations and/or chromosomal (multiple-gene) mutations. To a large extent, his question has set the agenda for mutagenesis research ever since. We explore historically the answers to this question, with special emphasis on recent developments in the field of mammalian cell mutagenesis. Studies indicate that ionizing radiation and many chemical mutagens/carcinogens induce both gene and chromosomal mutations; however, only certain genetic systems permit the recovery and analysis of both classes of mutations. Few chemical mutagens induce only gene mutations in mammalian cells; instead, most mutagens appear to induce both classes of mutations, with chromosomal mutations (especially multilocus deletions) predominating at high doses. These results have implications regarding the mechanisms of mutagenesis, the role of chromosomal mutations in carcinogenesis and hereditary disease, and the type of data required for risk assessment of physical and chemical mutagens/carcinogens.     

Effect of the uvs-2 allele of Neurospora crassa on the mutagenic potency of two N-hydroxylaminopurines and 2-aminopurine in the ad-3 forward-mutation test

The mutagenic potencies of 3 purine analogs were determined in the ad-3 forward-mutation test in growing cultures of heterokaryon 59 (H-59), a nucleotide excision repair-deficient (uvs-2/uvs-2) 2-component heterokaryon of Neurospora crassa. Two N-hydroxylaminopurines, 2-amino-6-N-hydroxylaminopurine (AHA) and 6-N-hydroxylaminopurine (HAP), were potent and strong mutagens, respectively, whereas 2-aminopurine (AP) was a moderate mutagen. Dose-response curves showed that AHA and HAP were about equally mutagenic at low doses but that AHA was more mutagenic than HAP at high doses. Comparison of these results in H-59 with our earlier results in heterokaryon 12 (H-12) of N. crassa, which is identical to H-59 except for being DNA-repair-proficient (uvs-2+/uvs-2+), shows that the defect in nucleotide excision repair due to uvs-2 has little or no effect on the mutagenic potencies of these 3 purine analogs. Therefore, the nucleotide excision-repair pathway in N. crassa that is deficient in H-59 does not appear to have a major role in the repair of pre-mutational lesions induced by these 3 purine analogs. On the other hand, based on the controls of these experiments, the frequency of spontaneous ad-3 mutants was 4 greater in H-59 than in H-12. This result suggests that the nucleotide excision-repair pathway in N. crassa that is inactivated by the uvs-2 mutation has a major role in the repair of lesions that would lead to spontaneous mutation at the ad-3+ region if they were not repaired.     

Isolation and characterization of quinoline-degrading bacteria from subsurface sediments.

Two gram-negative, motile bacteria isolated from deep subsurface sediments mineralized the nitrogen-containing polyaromatic hydrocarbon quinoline under aerobic conditions and transformed quinoline to soluble intermediates under anaerobic conditions. Many aromatic compounds were also able to serve as the sole source of carbon and energy under aerobic conditions. Rapid aerobic mineralization of quinoline at concentrations as low as 0.002 microgram ml-1 indicates that these organisms possess a high-affinity uptake and utilization system, which may reflect the oligotrophic nature of deep subsurface environments. Both bacteria harbored four plasmids of identical size, ranging from 50 to 440 kilobases.     

Effect of the homokaryotic state of the uvs-2 allele in Neurospora crassa on formaldehyde-induced killing and ad-3 mutation

Formaldehyde was tested for its killing and mutagenic activities in the ad-3 forward-mutation test in Neurospora crassa. The test was conducted in 3 two-component heterokaryons (dikaryons) of N. crassa in order to determine the effect of the uvs-2 allele, which causes a defect in nucleotide excision repair, on formaldehyde-induced killing and the induction of ad-3 mutants. These dikaryons were homokaryotic for uvs-2+ (H-12), homokaryotic for usv-2 (H-59), and heterokaryotic for uvs-2 (H-71). Formaldehyde induced killing and ad-3 mutants in H-12, but the presence of uvs-2 in the homokaryotic state (H-59) resulted in a 9-fold increase in killing and a 40-fold increase in the induction of ad-3 mutants. This increased sensitivity to formaldehyde-induced killing and mutation conferred by uvs-2 in the homokaryotic state (H-59 vs. H-12) is similar to that noted by others in Escherichia coli. Salmonella typhimurium and Saccharomyces cerevisiae. The dikaryon heterokaryotic for uvs-2 (H-71) has the same sensitivity to formaldehyde-induced ad-3 mutation as H-12, indicating that uvs-2 is recessive to uvs-2+.     

2-Amino-N6-hydroxyadenine induces gene/point mutations and multiple-locus mutations, but not multilocus deletion mutations, in the ad-3 region of a two-component heterokaryon of Neurospora crassa.

The mutagenicity of 2-amino-N6-hydroxyadenine (AHA) has been studied in Neurospora crassa by treating a two-component heterokaryon (H-12) and recovering specific-locus mutations induced in the ad-3 region. This assay system permits the identification of ad-3A and/or ad-3B mutants resulting from gene/point mutations, multilocus deletion mutations, and multiple-locus mutations of various genotypes, involving one or both loci. Genetic characterization of the ad-3 mutants recovered from experiments with AHA in H-12 shows that 98.9% (270/273) of the ad-3 mutants are gene/point mutations (ad-3R), 1.1% (3/270) are unknowns, and none is a multilocus deletion mutation (ad-3IR). Among the gene/point mutations, 3.3% (9/273) are multiple-locus mutations (gene/point mutations with a closely-linked recessive lethal mutation [ad-3R + RLCL]). Another 25.3% (69/273) are multiple-locus mutations with a recessive lethal mutation located elsewhere in the genome [ad-3R + RL]. Heterokaryon tests for allelic complementation among the ad-3BR mutants showed that 90.8% (139/153) of the mutants were complementing, and 20.3% (31/153) were leaky. In addition, 32.5% (38/117) of the ad-3AR mutants were leaky. These data are consistent with the hypothesis that AHA produces specific-locus mutations in the ad-3 region of N. crassa by base-pair substitution. The data from the present experiments are compared with the data for 2-aminopurine (2AP)-induced ad-3 mutants in H-12 (de Serres and Brockman, 1991). Whereas, 2AP is a weak mutagen in H-12, AHA is extremely potent (Brockman et al., 1987). In contrast with 2AP, AHA induces ad-3 mutants exclusively by gene/point mutation in H-12. We conclude that whereas AHA induces ad-3 mutants predominantly by AT to GC base-pair transitions, 2AP induces ad-3 mutants by a wide variety of mechanisms including: (1) AT to GC and GC to AT base-pair transitions, (2) frameshift mutations, (3) other, as yet unidentified, intragenic alterations, (4) small multilocus deletion mutations, and (5) multiple-locus ad-3R mutations with closely linked recessive lethal mutations.     

Qualitative differences in the spectra of genetic damage in 2-aminopurine-induced ad-3 mutants between nucleotide excision-repair-proficient and -deficient strains of Neurospora crassa.

The mutagenic effects of 2-aminopurine (2AP) have been compared in the adenine-3 (ad-3) region of two-component heterokaryons of Neurospora crassa: nucleotide excision repair-proficient (uvs-2+/uvs-2+) heterokaryon 12 (H-12) and nucleotide excision repair-deficient (uvs-2/uvs-2) heterokaryon 59 (H-59). This forward-mutation, morphological and biochemical, specific-locus assay system permits the recovery of ad-3A and/or ad-3B mutants in 3 major classes: gene/point mutations, multilocus deletion mutations, and unknowns, and 3 different subclasses of multiple-locus mutations. Previous studies (Brockman et al., Mutation Res., 218 (1989) 1-11) showed that 2AP treatment of growing cultures of H-12 and H-59 gave no difference between ad-3 forward-mutation frequencies over a wide range of 2AP concentrations in each strain. In the present experiments, genetic analyses of ad-3 mutants recovered from these experiments has demonstrated qualitative differences between the spectra of the 3 main classes of ad-3 mutations. In H-12, 84.2% (203/241) resulted from gene/point mutation, 11.6% (28/241) from multilocus deletion mutation, and 4.1% (10/241) were unknowns. In contrast, in H-59, 43.0% (99/230) resulted from gene/point mutation, 55.7% (128/230) from multilocus deletion mutation, and 1.3% (3/230) were unknowns. In addition, quantitative differences were also found between the spectra of ad-3 mutations in 1 subclass of multiple-locus mutations, but not 2 additional subclasses. The first subclass consisted of 1.7% (4/241) and 9.6% (22/230) gene/point mutations with a closely linked recessive lethal mutation, in H-12 and H-59, respectively. The second two subclasses consisted of (a) 0.4% (1/241) and 0.4% (1/230) multilocus deletion mutations with a closely linked recessive lethal mutation, and (b) 13.3% (32/241) and 15.2% (35/230) gene/point mutations with a separate recessive lethal mutation elsewhere in the genome, in H-12 and H-59, respectively. Data from studies by others have shown that 2AP inhibits adenosine deaminase, resulting in nucleotide precursor pool inbalance, and that 2AP can saturate the mismatch repair system. As a consequence of either effect of 2AP, the spectrum of 2AP-induced mutation could include frameshift mutations and chromosome aberrations such as multilocus deletions in addition to base-pair substitutions. The defect in DNA repair due to the uvs-2 allele, which has been shown to be a deficiency in pyrimidine dimer excision (Worthy and Epler, 1974), most probably has some other excision-repair deficiency (Macleod and Stadler, 1986; Baker et al., 1991).(ABSTRACT TRUNCATED AT 400 WORDS)     

Kinetic studies of adenosine kinase from L1210 cells: a model enzyme with a two-site ping-pong mechanism

Purified adenosine kinase from L1210 cells displayed substrate inhibition by high concentrations of adenosine (Ado), ATP, and MgCl2. When incubated with ATP and MgCl2, the enzyme was phosphorylated, and the phosphorylated kinase transferred phosphate to adenosine in the absence of ATP and MgCl2. Substrate binding, isotope exchange, and kinetic studies suggested that the enzyme catalyzes the reaction by means of a two-site ping-pong mechanism with the phosphorylated enzyme as an obligatory intermediate. Among many possible pathways within this mechanism probably a random-bi ordered-bi route is the preferred sequence in which the two substrates, adenosine and MgATP, bind in a random order to form the ternary complex MgATP . E . Ado followed by the sequential dissociation of MgADP and AMP. Dissociation constants of various enzyme-substrate and enzyme-product complexes and the first-order rate constant of the rate-limiting step were estimated.