Virus-associated enzymes

S. Stirm Degradation of bacterial surface carbohydrates by virus-associated enzymes methodology and applications... 

The inhibitory activity of daunomycin and its structural analogues on viral oncogenesis by FLV and RSV, and on in vitro transformation by MSV (M) suggests that it is the activity of the virus-associated enzymes which is sensitive to these antibiotics. The RNA-dependent DNA polymerase of the virions is responsible for the synthesis of viral DNA. Table 16 shows how the reverse-transcriptase activity of MSV (M), FLV and RSV is inhibited by various daunomycin derivatives. 

Once inside the host cell, the vims must replicate its own nucleic acid. To do this, it often uses part of the normal synthesizing machinery of the host cell. If the vims is to continue its growth cycle, viral nucleic acid and viral protein must be propedy transported within the cell, assembled into the infective vims particle, and ultimately released from the cell. All of these fundamental processes involve an intimate utilization of both cellular and viral enzymes. Certain enzymes that are involved in this process are specifically supplied by the invading vims. It is this type of specificity that can provide the best basis for antiviral chemotherapy. Thus an effective antiviral agent should specifically inhibit the viral-encoded or virus-induced enzymes without inhibition of the normal enzymes involved in the biochemical process of the host cell. Virus-associated enzymes have been reviewed (2,3) (Table 1). 

Agents that inhibit virus-associated enzymes, such as DNA polymerases and others... 

Viruses are small infectious agents composed of a nucleic acid genome (DNA or RNA) encased by structural proteins and in some cases a lipid envelope. They are the causative agents of a number of human infectious diseases, the most important for public health today being acquired immunodeficiency syndrome (AIDS), hepatitis, influenza, measles, and vituses causing diarrhoea (e.g., rotavirus). In addition, certain viruses contribute to the development of cancer. Antiviral drugs inhibit viral replication by specifically targeting viral enzymes or functions and are used to treat specific virus-associated diseases. 

The complete complex of nucleic acid and protein, packaged in the virus particle, is called the virus nucleocapsid. Although the virus structure just described is frequently the total structure of a virus particle, a number of animal viruses (and a few bacterial viruses) have more complex structures. These viruses are enveloped viruses, in which the nucleocapsid is enclosed in a membrane. Virus membranes are generally lipid bilayer membranes, but associated with these membranes are often virus-specific proteins. Inside the virion are often one or more virus-specific enzymes. Such enzymes usually play roles during the infection and replication process. 

Screening. Primary screening is necessary to eliminate nonspecific hybridomas as soon as possible. Screening is also used to test the hybridoma culture supernatant for antibody reactivity and specificity. As an example, an Epstein-Barr virus associated protein is coated onto plastic ELISA plates. After incubation of hybridoma culture supernatant, secondary enzyme-labeled conjugate and chromogenic substrate, a colored product indicates a positive hybridoma. Alternatively, immunocytochemical screening can be used. It is preferable to test hybridomas when at least three-quarters of them are confluent. 

Cidofovir is an acyclic nucleotide analogue of the monophosphate of cytosine. When phosphoiylated by host cellular enzymes, the active compound cidofovir diphosphate has broad activity against the herpes viruses, including CMV, HSV 1 and 2, VZV, Epstein-Barr virus, and the BK polyomavirus. Cidofovir has primarily been used in the treatment of CMV retinitis in patients who have failed treatment with ganciclovir or foscarnet and in acyclovir-resistant herpes simplex infections. More recently, there is also a growing experience with the use of this medication in kidney transplant patients who have BK virus-associated nephropathy [31], although this interest has been dampened by significant toxicity and only modest clinical activity [32]... 

Acidity allows the viruses associated with this disease to flourish, particularly the Epstein-Barr virus. It overloads the detoxification mechanisms and shuts down production of critical enzymes, thereby preventing proper detoxification of toxins linked to the disease. It can interfere with the uptake of nutrients, leading to the deficiencies that are common in CFS. Furthermore, chronic acidity can reduce the functionality of critical glands, such as the adrenal glands, a frequent problem for sufferers of CFS. 

Living systems are built upon from molecular materials or nanostructures such as nucleic acids (DNA and RNA) and protein. They are 2 and 5 to 50 nm wide, respectively. They can be produced from the self-assembly or self-organization processes in the hving system itself or by chemical synthesis. DNA or RNA and associated enzymes and proteins or hpids can be self-assembled into 75 to 100 nm wide viruses. They can be further assembled into bacteria. Bacteria are 1 to 10 m in size, with thin, rubbery cell membrane surrounding the fluid (cytoplasm) and all genetic information needed to make copies of its own DNA. Viruses and bacteria cause many diseases. A white blood cell is about 10 jum big whereas all materials internalized by cells are smaller than 100 nm. 

Thus, the antiviral activity of the thiosemicarbazones has been correlated, in one study, with their ability to inhibit ribonucleotide reductase [8]. Methisazone inhibits the RNA-dependent DNA polymerase of Rous sarcoma virus (RSV) and inactivates its ability to transform chick-embryo cells [9], as well as inactivating herpes simplex [10] and some RNA viruses [11]. The activity against RSV is Cu-dependent [9], the Cu complex also being capable of inhibition of the polymerase [12]. Since these enzymes are zinc-dependent, interference with the role of zinc is possible. The inhibition of influenza virus-associated RNA transcriptase by... 

Oxford, J. S., 1973, Polypeptide composition of influenza B viruses and enzymes associated with the purified virus particles, J. Virol. 12 827-835. 

Substances that do not target the active site but display inhibition by allosteric mechanisms are associated with a lower risk of unwanted interference with related cellular enzymes. Allosteric inhibition of the viral polymerase is employed in the case of HIV-1 nonnucleosidic RT inhibitors (NNRTl, see chapter by Zimmermann et al., this volume) bind outside the RT active site and act by blocking a conformational change of the enzyme essential for catalysis. A potential disadvantage of targeting regions distant from the active site is that these may be subject to a lower selective pressure for sequence conservation than the active site itself, which can lower the threshold for escape of the virus by mutation. 

This enzyme is associated with the virions of RNA tumor viruses such as the Rous sarcoma virus (RSV). The enzyme has remarkable enzymatic activity in that it can catalyze several seemingly diverse steps in the synthesis of double-stranded DNA from the single-stranded RNA viral genome. The enzyme uses a tRNA for tryp-tophan as a primer to make a copy of DNA that is complementary to the viral RNA. The resulting RNA-DNA hybrid is converted to a double-stranded DNA molecule by ribon-uclease (RNase)H and DNA-dependent DNA polymerase activities that are intrinsic to reverse transcriptase. 

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