Enzyme最新文献

The Sendai virus P gene contains overlapping open reading frames (ORFs), and several unusual mechanisms are used to produce multiple proteins from all three ORFs of this gene. These include the use of a non-AUG start codon, leaky ribosomal scanning, what appears to be scanning-independent ribosomal initiation and/or ribosomal jumping, and a form of mRNA editing.

Ribosome binding to cellular eukaryotic mRNAs is proposed to occur by initial attachment at or near the mRNA 5' cap structure (m7 GppN, where N is any nucleotide) followed by scanning till the appropriate initiator AUG is encountered. A pivotal aspect of this model is the obligatory entry of the ribosomes at the 5' end of the mRNA (regardless if it contains a cap structure). Recent experiments, however, demonstrated that ribosomes can access certain mRNAs by internal binding to the 5'-untranslated region. This was most clearly demonstrated for members of the picornavirus family such as poliovirus and encephalomyocarditis virus. Further experiments suggest that other viral mRNAs and even cellular mRNAs may use similar mechanisms of ribosome binding. Here we describe some features of the poliovirus 5'-untranslated region and possible trans-acting factors that are involved in this mechanism of translation.

Rimantadine-resistant and -sensitive influenza A variants were assayed for their sialidase (neuraminidase, EC 3.2.1.18) activity. The kinetic parameters determined (pH optimum, stability against different pH values, thermal stability, activity on methylumbelliferyl-alpha-D-N-acetylneuraminic acid, N-acetylneuraminyl-lactose, fetuin and bovine submandibular gland mucin as substrates, Km with the former substrate, inhibition by two competitive inhibitors, and behavior towards amantadine) revealed the same results for both variants of the virus. Thus, it can be deduced that resistance to rimantadine does not influence the sialidase activity of influenza A virus.

Plasma activity and excretion of pancreatic (P) amylase in the rat was found to be negligible. In contrast, the excretion rate of salivary (S) amylase was substantial and variable, depending on diuresis. P-amylase had a higher isoelectric point, a greater sieving coefficient, and a shorter half-life than S-amylase. A bolus injection of 125I-labelled enzymes was followed by the appearance of 125I-labelled enzyme- as well as protein-free 125I activity in the urine. The enzyme loss was smaller and the fraction of protein-free 125I activity higher following injection of P-amylase. The affinity of P-amylase to paraffin oil exceeded that of S-amylase in partition experiments with water and paraffin oil in vitro. It is concluded that both renal filtration and reabsorption of P-amylase exceed those of S-amylase. This might be due to the higher lipophility of P-amylase in comparison to the salivary type.

Retroviruses, related retroposons, and several RNA viruses use translational frameshifting in the expression of their polymerase genes. We use retroviruses, particularly mouse mammary tumor virus, to illustrate the model which reflects our current understanding of these site-specific frameshifting events. The model has two components, a shifty sequence that facilitates ribosome slippage, and a second signal, either RNA secondary structure or an unfilled ribosomal A site, that stalls the ribosome and increases the probability of slippage at the shifty site.

Excess iron results in an increase in the translation of ferritin mRNA and a decrease in the stability of transferrin receptor (TfR) mRNA. These coordinate regulatory events are mediated by similar sequence/structure motifs that exist within the 5' untranslated region (5'UTR) of the ferritin mRNA and the 3'UTR of the TfR mRNA. We have referred to these motifs as iron-responsive elements (IREs). The IREs from both transcripts interact with a cytoplasmic protein that we have called the IRE-binding protein (IRE-BP). The activity but not the amount of the IRE-BP is dependent on the cellular iron status. The biochemical basis for the altered activity of the IRE-BP appears to be the reversible oxidation-reduction of one or more cysteines in the IRE-BP. The IRE-BP is a 90- to 95-kD cytosolic protein that has been purified to homogeneity by RNA affinity chromatography, and the cDNA corresponding to the IRE-BP has been molecularly cloned. Collectively, our data support a model in which the interaction between the IRE-BP and the ferritin IRE results in attenuation of translation, and similar interaction with TfR mRNA can protect the transcript from rapid degradation mediated by a rapid turnover determinant within the 3'UTR.

The c-fos proto-oncogene mRNA is extremely labile and is rapidly degraded within minutes after being transported to the cytoplasm of growth factor-stimulated fibroblasts. Analysis of the structural determinants controlling c-fos message decay reveals that this message contains at least two functionally independent elements that are responsible for its short half-life. One of these determinants is an AU-rich sequence present in the 3' untranslated region of the c-fos message, whereas the other determinant, which is structurally unrelated to the AU-rich element, is located within the c-fos protein-coding sequence. Both the c-fos AU-rich element and the coding region instability determinant appear to function by facilitating rapid removal of the c-fos poly(A) tail as a first step in the mRNA degradation process.

An attempt has been made to determine the intracellular distribution of the multiforms of the adenylate kinase (AK) isoenzymes in mammalian tissues, to shed some light on their physiological roles, especially in energy metabolism. The adenylate kinase zymograms obtained from isoelectric focusing yielded two typical isoform patterns: (1) with a pI greater than or equal to 9 and 8.6, specific for bovine skeletal muscle, heart, aorta and brain, and (2) with a pI = 7.9 and 7.1, specific for liver and kidney. Pattern (1) was attributed to the cytosolic isoenzyme (AK1) as demonstrated by immunostaining with anti-AK1. Pattern (2) was attributed to the mitochondrial isoenzyme (AK2). These results were largely confirmed by chromatofocusing experiments. The AK1 isoenzyme was partially purified from the cytosol fraction of bovine aortic smooth muscle and had an apparent Mr of 23.5 kilodaltons. Its kinetic features are discussed from a comparative standpoint. Finally, the human serum AK1 isoform was also detected by Western blotting with a monoclonal antibody directed against crystalline porcine muscle AK1. These results are to form the basis of further studies on the 'aberrant' adenylate kinase isoenzyme from the serum of Duchenne muscular dystrophics.

Extreme degrees of hypoxanthine phosphoribosyltransferase (HPRT) deficiency in man are associated with gross sex-linked neurological dysfunction, gout and urinary stones (the Lesch-Nyhan or 'complete HPRT-deficiency' syndrome). The less severe degrees of enzyme deficiency (sex-linked recessive gout and/or urolithiasis or the 'partial HPRT-deficiency' syndrome) may be associated with minor neurological manifestations. Whole body purine synthesis de novo is accelerated in both these groups of patients. A strain of mice with an experimentally produced mutation at the HPRT locus showed some residual 'apparent HPRT activity' in brain, liver, testicular, splenic, kidney and ovarian tissues but not in erythrocyte haemolysates. The mutation removes exons 1 and 2 of the coding region of the gene together with the promotor and about 10 kb of upstream sequence from the gene. It is therefore possible that the observed 'apparent HPRT activity' in these mice is due to the operation of an alternative metabolic pathway. Purine synthesis de novo was markedly accelerated in their brain, testicular, splenic and kidney tissues. It was not accelerated in the liver tissue of male mice hemizygous for the mutation and the degree of acceleration in the female homozygotes only just reached statistical significance at the p = 0.02 level. This observation casts doubt on the importance of modulations in the rate of hepatic purine synthesis de novo as a mechanism for maintaining a steady supply of purines for translocation to other organs.