Reverse transcriptase other viruses

AZT works by specifically blocking DNA synthesis carried out by HIV reverse transcriptase. Other related compounds are also being tested to see if they specifically affect HIV reverse transcriptase. Such compounds might have equivalent antiviral effects. If they have fewer side effects than AZT, they may be even more effective in treating HIV-infected individuals. Two additional antivirals related to AZT have recently been approved for anti-HIV therapy, dideoxy-inosine (DDI) and dideoxycytosine (DDC). These drugs are predominantly recommended for individuals who cannot tolerate AZT, or for whom AZT has ceased to be effective although they are effective against HIV, they do have side effects. Nevertheless, they may be important because AZT does not indefinitely reduce the amount of virus in HIV-infected individuals. 

The process of transcription can also occur in reverse, from RNA to DNA, when the sequence coded in RNA is transcribed into antisense DNA by reverse transcriptase enzymes first discovered by Temin and Mizutani [18] and Baltimore [19]. Further integrase enzymes insert the DNA sequence into the native DNA. This is the mechanism by which viruses infect their host organisms and can be stopped by antiviral drugs, such as azidothymine (AZT) that inhibits F1IV reverse transcriptase. Other processes such as the extension of telomeres at the ends of chromosomes by telomerase, to control programmed cell death, use the same mechanism. 

The viruses responsible for AIDS are human immunodeficiency virus 1 and 2 (HIV 1 and HIV 2) Both are retroviruses, meaning that their genetic material is RNA rather than DNA HI Vs require a host cell to reproduce and the hosts m humans are the T4 lymphocytes which are the cells primarily responsible for inducing the immune system to respond when provoked The HIV penetrates the cell wall of a T4 lymphocyte and deposits both its RNA and an enzyme called reverse transcriptase inside There the reverse transcriptase catalyzes the formation of a DNA strand that is complementary to the viral RNA The transcribed DNA then serves as the template from which the host lymphocyte produces copies of the virus which then leave the host to infect other T4 cells In the course of HIV reproduction the ability of the T4 lymphocyte to reproduce Itself IS compromised As the number of T4 cells decrease so does the body s ability to combat infections... 

Reverse transcriptase (from avian or murine RNA tumour viruses) [9068-38-6] [EC 2.7.7.49]. Purified by solubilising the virus with non-ionic detergent. Lysed virions were adsorbed on DEAE-cellulose or DEAE-Sephadex columns and the enzyme eluted with a salt gradient, then chromatographed on a phosphocellulose column and enzyme activity eluted in a salt gradient. Purified from other viral proteins by affinity chromatography on a pyran-Sepharose column. [Verna Biochim Biophys Acta 473 1 7977 Smith Methods Enzymol 65 560 1980 see commercial catalogues for other transcriptases.]... 

Other nucleosides such as 2, 3 -dideoxyinosine (ddl) also block the action of reverse transcriptase and are often combined with AZT in drug cocktails. Using a mixture of drugs makes it more difficult for a virus to develop resistance than using a single drug. 

The first lead compounds for non-nucleoside reverse transcriptase (RT) inhibitors (NNRTl) were discovered about 15 years ago (Pauwels et al. 1990 Merluzzi et al. 1990 Goldman et al. 1991 De Clercq 1993 Riibsamen-Waigmann et al. 1997). Since then they have become an important ingredient of the dmg combination schemes that are currently used in the treatment of human immunodeficiency virus type 1 (HlV-1) infections. Starting from the HEPT and TIBO derivatives, numerous classes of compounds have been described as NNRTIs. Four compounds (nevirapine, delavirdine, efavirenz and etravirine) have so far been approved for clinical use and several others are the subject of clinical trials (Balzarini 2004 Stellbrink 2007). 

Autoradiography The detection of radioactive molecules (eg, DNA, RNA, protein) by visualization of their effects on photographic film. Bacteriophage A virus that infects a bacterium. Blunt-ended DNA Two strands of a DNA duplex having ends that are flush with each other. cDNA A single-stranded DNA molecule that is complementary to an mRNA molecule and is synthesized from it by the action of reverse transcriptase. 

After the virus has attached to CD4 and chemokine receptors, another viral glycoprotein (gp41) assists with viral fusion to the cell and internalization of the viral contents. The viral contents include single-stranded RNA, an RNA-dependent DNA polymerase (also known as reverse transcriptase), and other enzymes. Using the single-stranded viral RNA as a template, reverse transcriptase synthesizes a complementary strand of DNA. The single-stranded viral RNA is removed from the newly formed DNA strand by ribonuclease H, and reverse transcriptase completes the synthesis of double-stranded DNA. The... 

Enzymes in viruses We have stated that virus particles do not carry out metabolic processes. Outside of a host cell, a virus particle is metabolically inert. However, some viruses do contain enzymes which play roles in the infectious process. For instance, many viruses contain their own nucleic acid polymerases which transcribe the viral nucleic acid into messenger RNA once the infection process has begun. The retroviruses are RNA viruses which replicate inside the cell as DNA intermediates. These viruses possess an enzyme, an RNA-dependent DNA popo called reverse transcriptase, which transcribes the information in the incoming RNA into a DNA intermediate. It should be noted that reverse transcriptase is unique to the retroviruses and is not found in any other viruses or in cells. 

Trophoblastin, therefore, has been named interferon-tau (IFN-x), and is classified as a type I interferon. There are at least three or four functional IFN-x genes in sheep and cattle. The molecule displays a molecular mass of 19 kDa and an isoelectric point of 5.5-5.7, in common with other type I interferons. Interestingly, the molecule can also promote inhibition of reverse transcriptase activity in cells infected with the HIV virus. 

The enzyme reverse transcriptase, as well as some other enzymes used in virus replication the gene that codes these enzymes is the pol gene. The other viral enzymes specified by the pol gene are protease and integrase. Protease is involved in muturation of viral proteins as the virus buds out from the cell, and integrase is responsible for integration of the viral DNA into the cell s chromosomal DNA. 

In addition to drugs such as AZT, other antivirals targeted at reverse transcriptase are also being developed. AZT (and its relatives DDI and DDC) inhibits HIV replication by mimicking normal building blocks of DNA and being selectively incorporated by reverse transcriptase into viral DNA as opposed to cellular DNA. Viral DNA that has incorporated these compounds cannot be completed, and virus replication is aborted. Other compounds have been developed that directly inhibit the activity of HIV reverse transcriptase, with relatively little effect on cellular DNA polymerases. The net effect of these compounds also is to selectively inhibit HIV replication. One class of reverse transcriptase inhibitors currently being tested is referred to as TIBO inhibitors. 

Drugs are also used to inhibit the enzymatic reactions of foreign pathogens that enter the human body. An example is the use of reverse transcriptase inhibitor and protease inhibitor for combating the human immunodeficiency virus (HIV), as shown in Exhibit 2.12. Some new inhibitors are used to block HIV from attaching to the human cell, CD4, thus stopping replication and infection of other cells, as presented in Exhibit 2.13. 

The rapid spread of acquired immune deficiency syndrome (AIDS) has prompted numerous efforts to develop therapeutic agents against the human immunodeficiency virus type 1 (HIV-1) [2351. Efforts have focused on inhibition of the virally encoded reverse transcriptase (RT) enzyme, which is responsible for the conversion of retroviral RNA to proviral DNA. The nucleoside RT inhibitors 3 -azidothymidine (AZT) and dideoxyinosine (ddl) have proven to be clinically useful anti HIV-1 agents [236], but due to their lack of selectivity versus other DNA polymerases, these compounds are flawed by their inherent toxi-... 

The reverse transcriptase enzyme (RT) is the primary enzyme responsible for the conversion of the viral single-strand RNA to the double-strand DNA. The reverse transcriptase enzyme is a component of the virion and is encoded by the pol gene. The RT is manufactured in the HIV-infected cells as a gag-pol fusion polyprotein. The RT is not the only enzyme necessary for the translation of RNA to DNA. The other enzymes for this conversion include RNA-dependent DNA polymerase, DNA-dependent DNA polymerase, and RNase H (Gilboa and Mitra, 1978 Prasad and Gogg, 1990). The reverse transcriptase enzyme has a high error rate (1 in 2000 bases), which produces higher incidents of mutation. Some of these mutations make the virus resistant to NNRTI treatment. 

Antibodies against the virus but also amantadine and derivatives, interfere with host cell penetration. There are nucleoside analogues such as aciclovir and ganciclovir, which interfere with DNA synthesis, especially of herpes viruses. Others like zidovudine and didanosine, inhibit reverse transcriptase of retroviruses. Recently a number of non-nucleoside reverse transcriptase inhibitors was developed for the treatment of HIV infections. Foscarnet, a pyrophosphate analogue, inhibits both reverse transcriptase and DNA synthesis. Protease inhibitors, also developed for the treatment of HIV infections, are active during the fifth step of virus replication. They prevent viral replication by inhibiting the activity of HIV-1 protease, an enzyme used by the viruses to cleave nascent proteins for final assembly of new vi-rons. 

Like all other retroviruses, human immunodeficiency virus type 1 (HIV-1) contains the multifunctional enzyme reverse transcriptase (RT). Retroviral RTs have a DNA polymerase activity that can use either an RNA or a DNA template and an RNase H activity. HIV-1 RT is essential for the conversion of single-stranded viral RNA into a linear double-stranded DNA that is subsequently integrated into the host cell chromosomes [1-4]. In this conversion process HIV-1 RT catalyzes the incorporation of approximately... 

Brennan TM, Taylor DL, Bridges CG, Leyda JP, Tyms AS. The inhibition of human immunodeficiency virus type 1 in vitro by a non-nucleoside reverse transcriptase inhibitor MKC-442 alone and in combination with other anti-HIV compounds. Antiviral Res 1995 26 173-187. 

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