Lymphocyte cytochrome P450 expression: inducibility studies in male Wistar rats.

The cytochrome P450 system plays a key role in the metabolism of endogenous and exogenous compounds. The system is distributed widely in body tissues, with the highest concentration of the enzymes found in liver hepatocytes. Extrahepatic expression of the P450 system has been documented in the lung, pancreas and kidney, and the enzymes are induced by many disease states, including diabetes mellitus and cancer. Little attention has been paid to the expression and inducibility of the system in peripheral blood lymphocytes. In this study, specific P450 inducers are administered in vivo to male Wistar rats. The expression and in vivo induction of the P450 isoforms CYP2B, CYP2E, CYP3A and CYP4A in liver and lymphocyte samples is determined using Western blot analysis. Following in vivo induction, the lymphocyte P450 proteins showed an average three-fold increase in expression (0.003-0.005 microg P450/microg microsomal protein), compared to the control lymphocyte samples. Expression in the induced lymphocyte samples was up to 11-fold lower than that in the induced liver samples, as expected. These results indicate that lymphocytes may provide a relatively simple method by which to monitor the P450 profile in human subjects.

Influence of habitual diet on antioxidant status: a study in a population of vegetarians and omnivores.

BACKGROUND: Antioxidant status can be used as a biomarker to assess chronic disease risk and diet can modulate antioxidant defence. OBJECTIVE: To examine effects of vegetarian diet and variations in the habitual intakes of foods and nutrients on blood antioxidants. SUBJECTS AND SETTING: Thirty-one vegetarians (including six vegans) and 58 omnivores, non-smokers, in Northern Ireland. DESIGN: A diet history method was used to assess habitual diet. Antioxidant vitamins, carotenoids, uric acid, zinc- and ferric-reducing ability of plasma (FRAP) were measured in fasting plasma and activities of glutathione peroxidase (GPX), superoxide dismutase (SOD) and glutathione S-transferase (GST) and level of reduced glutathione (GSH) were measured in erythrocytes. RESULTS: Vegetarians had approximately 15% higher levels of plasma carotenoids compared with omnivores, including lutein (P< or =0.05), alpha-cryptoxanthin P< or =0.05), lycopene (NS), alpha-carotene (NS) and beta-carotene (NS). The levels/activities of all other antioxidants measured were similar between vegetarians and omnivores. Total intake of fruits, vegetables and fruit juices was positively associated with plasma levels of several carotenoids and vitamin C. Intake of vegetables was positively associated with plasma lutein, alpha-cryptoxanthin, alpha-carotene and beta-carotene, whereas intake of fruits was positively associated with plasma beta-cryptoxanthin. Intake of tea and wine was positively associated with FRAP value, whereas intake of herbal tea associated positively with plasma vitamin C. Intakes of meat and fish were positively associated with plasma uric acid and FRAP value. CONCLUSIONS: The overall antioxidant status was similar between vegetarians and omnivores. Good correlations were found between intakes of carotenoids and their respective status in blood.

Quantification of DNA damage and repair in amino acid auxotrophs and UV-sensitive mutants of Aspergillus nidulans using an ELISA.

An ELISA used to investigate DNA repair in mammalian cells has been adapted to investigate mutagen-induced DNA damage and repair in protoplasts of Aspergillus nidulans. The assay shows a reduced rate of repair of DNA damage in methionine and arginine auxotrophs (methG and argB), which were shown previously to be hypersensitive to UV radiation and chemical mutagens. The assay also showed a considerably reduced ability to repair mutagen-induced damage in the uv-sensitive mutants uvsB and uvsH. The increased sensitivity of amino acids auxotrophs to mutagens is, therefore, correlated with a reduced capacity to repair mutagen-induced DNA damage.

Germinating conidiospores of Aspergillus amino acid auxotrophs are hypersensitive to heat shock, oxidative stress and DNA damage.

Germinating conidiospores (conidia) of Aspergillus nidulans amino acid-requiring strains are hypersensitive to heat, oxidative stress, UV radiation and chemical mutagens when compared with other strains. They also showed an increased mutation rate. Sensitivity to stress conditions has been correlated with an abnormal RAS/cAMP pathway in mutants of S. cerevisiae. We suggest that the RAS/cAMP pathway is defective in germinating conidia of Aspergillus amino acid auxotrophs and that this is responsible for suppressing DNA repair and conferring sensitivity to oxidative stress and heat shock.

DNA damage and mutation: contributors to the age-related alterations in T cell-mediated immune responses?

The genetic material of our cells is susceptible to damage by a wide variety of extrinsic and intrinsic entities. The amount of genetic damage accumulated in vivo will depend upon an individual's ability to defend against and/or repair DNA damage. T cells in vivo have been shown to accumulate DNA damage and mutations over time. The accumulation of such genetic damage will occur in T cells possessing a 'naive' or a 'memory' phenotype. Since T cells are required to undergo extensive clonal expansion upon antigenic stimulation, DNA damage and mutations may result in: a failure of T cells to proliferate, because of DNA damage-mediated cell cycle arrest; decreased rates of proliferation, as a consequence of selection in vivo against cells carrying certain mutations and/or apoptosis, triggered by critical levels of DNA damage. Thus, when T cells, containing critical levels of genetic damage, are required to undergo rapid clonal expansion in the presence of antigen, insufficient numbers of T cells may be produced and so the immune response would be sub-optimal. In this paper the possible contribution of DNA damage and/or mutation to the age-related alterations in T cell-mediated immune responses will be discussed.

Gene technology-based antimetabolite design: the use of an in vitro protein expression system to facilitate antimetabolite design for virally-induced human diseases and malignant conditions.

A precondition for the chemotherapeutic treatment of a variety of virally-induced human diseases and malignant conditions is a highly selective interaction of the drug molecule to be used with it's biological target. To ensure the development of novel, effective drugs, it is essential that the biological target is well characterised with regard to it's structure and activity. Such characterisation relies upon adequate amounts of pure target being available. One of the most important enzymatic importers for antimetabolites is the enzyme thymidine kinase. In this article an in vitro protein expression system is described which facilitates the production of milligram amounts of pure and biologically active thymidine kinase, from a number of important biological sources. Results have shown that the in vitro produced enzyme has the exact biochemical propeties of the in vivo enzyme. Thus the in vitro protein expression system is an ideal vechicle to facilitate an in depth investigation of the enzyme's biological properties.

Mitochondrial DNA polymorphism: its role in longevity of the Irish population.

The mtDNA genome has been implicated as playing a pivotal role in determining the longevity and success of the human lifespan. A PCR-RFLP methodology was used to identify polymorphic restriction enzyme sites within a 2643 bp region of the mtDNA genome and a table of genetic haplotypes for a healthy aged and a younger control cohort of patients was constructed. Forty-six different mtDNA haplotypes and 11 groups of related haplotypes were identified across the two age groups but statistical analysis failed to show any significant associations. The European J haplogroup, previously reported to be associated with longevity, was not found at an increased frequency within the Irish aged population (P=0.36). However, the haplotypes comprising the J haplogroup could be differentiated into two distinct branches by the presence or absence of the two polymorphic restriction sites, 16,389g and 16,000g. The branch of haplotypes defined by 16,389g displayed a significant increased frequency in the aged samples (8%) compared to the controls (1%), P=0.015. Inversely, the branch of haplotypes defined by 16,000g displayed a significant decreased frequency in the aged samples (4%) compared to the controls (13%), P=0.011. The polymorphism (mt5178A) associated with longevity in the Japanese was not found in the Irish population, while the polymorphism (mt9055A) associated with successful ageing in the French centenarians was found at an increased frequency in the Irish aged population (9%) compared to the younger control group (5%), but failed to reach a level of statistical significance, P=0.164.

Effect of a dCTP:dTTP pool imbalance on DNA replication fidelity in Friend murine erythroleukemia cells.

Nucleotide pool imbalances have been reported to affect the fidelity of DNA replication and repair in prokaryotic and eukaryotic cells. We have reported previously that the mutagen-hypersensitive thymidine kinase (TK)-deficient Friend erythroleukemia (FEL) cells (subclones 707BUF and 707BUE), have a more than sixfold increase in the dCTP:dTTP pool ratio when compared to that of wild-type, TK-positive (TK(+)) clone 707 cells. In this study we present the results of an investigation of the effect of the dCTP:dTTP pool imbalance on the accuracy of DNA replication within 707BUF cells. We examined the spontaneous mutation spectra occurring at the adenine phosphoribosyltransferase (aprt) locus within clone 707 (TK(+)) and 707BUF (TK(-)) FEL cells. Mutations recovered at the aprt locus in FEL cells comprised: base substitutions (43:73), frameshifts (14:13.5), and deletions (43:13.5) [clone 707 (TK(+)):707BUF (TK(-)), respectively, expressed as percentages]. A comparison of the mutation spectra obtained for the two cell lines did not reveal any significant increase in misincorporation of dCTP, the nucleotide in excess, in 707BUF (TK(-)) cells, during DNA replication synthesis. These data suggest that the dCTP:dTTP pool imbalance does not alter the fidelity of DNA replication synthesis in 707BUF (TK(-)) FEL cells. Rather, the predominance of GC --> AT transitions (53%) in the 707BUF (TK(-)) spectrum may reflect a reduced efficiency of repair by uracil DNA glycosylase of uracil residues within these cells.

An investigation of antioxidant status, DNA repair capacity and mutation as a function of age in humans.

We are constantly exposed, throughout life, to a wide variety of extrinsic and intrinsic agents which have the potential to damage cellular biomolecules, including DNA. Imperfections in cellular defence systems which protect against the fixation of DNA damage can lead to an accumulation of mutations which on their own, or in combination with other age-related changes, may contribute to ageing and the development of age-related pathologies. We have previously reported an increase in frequency of mutation with age in human lymphocytes taken from healthy males in the age groups, 35-39, 50-54 and 65-69 years. In this article we report on the findings of a recent study which was designed to assess whether the age-related increase in frequency of mutation was due to a decreased efficacy of the defence systems against ROS-induced DNA damage, namely antioxidant status and DNA repair processes, in the same study subjects. In vivo antioxidant status was assessed in each of the study subjects by measuring blood levels of; superoxide dismutase (SOD; EC 1.15.1.1), glutathione peroxidase (GPx; EC 1.11.1.9), catalase (EC 1.11.1.6), caeruloplasmin (CPL), uric acid and bilirubin. We did not find any statistically significant differences in the mean levels of these antioxidants between the three different age groups. To investigate the efficacy of DNA repair processes against ROS-induced DNA damage, an ELISA was used to quantitate DNA damage (as % single-stranded DNA; %SS-DNA) at various times following treatment of peripheral blood lymphocytes with hydrogen peroxide (H2O2). The results of this part of the study showed that in untreated lymphocytes, basal levels of %SS-DNA were significantly higher in individuals from the 65-69 years age group compared to the 35-39 years age group (p = 0.039, 0.0013; at 5% level of significance). No significant differences were found in H2O2 susceptibility with age immediately following treatment (p = 0.71, 1.00; at 5% level of significance) but a consistent and significant increase was observed in %SS-DNA remaining 90 min post-treatment in lymphocytes from subjects in the 65-69 years age group, compared to %SS-DNA present in lymphocytes from the 35-39 years age group (p = 0.013, 0.024; at 5% level of significance). The results of this study suggest that the age-related increase in frequency of mutations is not contributed to by alterations of in vivo antioxidant status with age but is by a decreased efficacy of the repair of ROS-induced DNA damage with age. The biological implications of somatic mutations in the ageing process are discussed.

An investigation of the cytotoxic and mutagenic potential of low intensity laser irradiation in Friend erythroleukaemia cells.

The purpose of this study was to investigate the cytotoxic and genotoxic potential of low intensity laser irradiation (660 nm, 12 mW, 5 kHz) on mammalian cells. Thymidine kinase (TK)-positive and TK-deficient Friend erythroleukaemia (FEL) cells, clone 707 and subclone 707BUF respectively, were used in this investigation. Following irradiation of exponentially growing cells in suspension at doses of 2 and 20 J/cm2 a number of sensitive bioassays were used to facilitate the detection of laser-induced mutations, DNA damage and cell killing. Mutations were assessed by the examination of chromosome spreads, the determination of micronucleus frequency and by the determination of the mutant frequency at the hypoxanthine-guanine phosphoribosyltransferase (hgprt) locus. DNA damage was quantified using a sensitive ELISA. The cytotoxic effect of laser irradiation was assessed using a cloning assay. The results of this investigation did not show any significant increase in mutation frequency, DNA damage or cell survival in the laser-irradiated cells, compared to sham-irradiated controls. The lack of any demonstrable cytotoxic and genotoxic effects of low intensity laser irradiation on mammalian cells in culture would support it as being a safe modality for clinical use.

Antioxidant status in humans after exposure to hyperbaric oxygen.

Hyperbaric oxygen (HBO) treatment (i.e., exposure to 100% oxygen at a pressure of 2.5 atmosphere absolute (ATA) for a total of 3 x 20 min periods) of human subjects induced DNA damage in the alkaline comet assay with leukocytes and protected against the DNA damaging effects of subsequent in vivo HBO exposures. Furthermore, blood taken 24 h after the first HBO was well protected against the in vitro induction of genotoxic effects by hydrogen peroxide. To investigate the mechanisms which led to this apparent adaptive response, we determined the antioxidant status of blood from subjects before and after HBO. We did not find differences in the plasma concentrations of the antioxidant vitamins A, C and E after HBO treatment. HBO had also no effect on the 'antioxidant power' of the plasma as measured with the FRAP-assay or on the concentration of reduced glutathione determined in the plasma or in lymphocytes. Red cell concentrate activities of superoxide dismutase, catalase, glutathione peroxidase were not influenced by HBO. In contrast, synthesis of the heat shock protein HSP70 which has been implicated to play an important role in cellular protection against oxidative stress, was significantly induced in lymphocytes after a single HBO treatment. To investigate whether intake of antioxidants may protect against HBO-induced DNA damage, we supplemented subjects with vitamin E (800 mg for 7 days) or with N-acetylcysteine (400 mg, 1 h before the HBO treatment). However, these supplementations did not influence the induction of DNA damage by HBO.

In vivo antioxidant status, DNA damage, mutation and DNA repair capacity in cultured lymphocytes from healthy 75- to 80-year-old humans.

The accumulation of damage to cellular biomolecules, including DNA, over time may play a significant role in the aetiology of the ageing process. We have previously quantified DNA damage and mutation within cultured lymphocytes from healthy human male subjects in three different age groups (35-39, 50-54 and 65-69 years). The results of that study showed an age-related increase in DNA damage and mutations in lymphocytes. In addition, an age-related decrease in the capacity of the lymphocytes to repair H2O2-induced DNA damage was found. In this article, we report the findings of an extension to the earlier study. Thirty-one generally healthy male and female subjects between the ages of 75 and 80 years were recruited. Using a number of bioassays, we were able to determine; basal levels of DNA damage (for 18 subjects) and mutant frequency at the hypoxanthine phosphoribosyltransferase (hprt) gene locus (for 16 subjects) within cultured lymphocytes. In addition, in vivo antioxidant status (for all study subjects) and the capacity of lymphocytes to repair H2O2-induced DNA damage (for 18 subjects) were also assessed. The results obtained showed: that the mean basal level of DNA damage in lymphocytes from subjects in the 75- to 80-year age group (12.6 +/- 4.7%) was similar to that of the 35- to 39-year age group (13.3 +/- 3.3%), p = 0.42 (Mann-Whitney); there was no significant difference between log mean mutant frequency at the hprt gene locus in lymphocytes from the 75- to 80-year age group (0.31 +/- 0.33) compared to that observed in the 35- to 39-year age group (0.24 +/- 0.21; Student's t-test, t = 0.68, p > 0.05). Levels of the antioxidants glutathione peroxidase (GPx EC 1.11.1.9), catalase (CAT; EC 1.11.1.6) and caeruloplasmin (CPL; EC 1.16.3.1) were significantly elevated in the 75- to 80-year age group, compared to the 35- to 39-, 50- to 54- and 65- to 69-year age groups. Levels of bilirubin (BR) were reduced in the 75- to 80-year age group, the decrease being contributed by the female subjects. No differences in levels of superoxide dismutase (SOD; EC 1.15.1.1) or uric acid (UA) were found between the 4 age groups. Following treatment of lymphocytes with H2O2, we did not find any difference in the susceptibility of lymphocytes to DNA damage in the 75- to 80-year age group, compared to the other age groups. The DNA repair capacity in lymphocytes from individuals in the 75- to 80-year age group was similar to that of the 35- to 39-year age group, for all time points assessed. These results highlight the importance of DNA repair processes and antioxidant defence systems for maintaining genomic stability in vivo.

An investigation of mutation as a function of age in humans.

An accumulation of mutations on their own or together with other age-related changes may contribute to aging and the development of age-related pathologies. The aim of this investigation was to assess the extent of DNA mutations as a function of age in humans. The mutant frequency (MF) at the hypoxanthine-guanine phosphoribosyl-transferase (hgprt) locus was assessed in lymphocytes isolated from male volunteers in each of three age groups (35-39, 50-54 and 65-69 years). Results show that the mean MF in the 65-69 years group was approximately twice that in the 35-39 and 50-54 years groups (4.1/10(6) cells, 1.9/10(6) cells and 1.79/10(6) cells, respectively) increasing by about 1.33% per year, after 54 years. In addition, there was an increased frequency of chromosomal aberrations in the 65-69 years group compared to the other two age groups. The results of this investigation show an increase in DNA mutations in cultured human lymphocytes with age. Factors which may influence the extent of DNA damage in human lymphocytes are discussed.

Effect of dietary intake and lifestyle factors on in vivo mutant frequency at the HPRT gene locus in healthy human subjects.

This paper describes the results of a study designed to assess the effects of a variety of dietary and lifestyle factors on background levels of mutant frequency (MF) at the hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene locus in humans. Eighty-three healthy and free-living subjects (aged 20-80 yr; 61 males and 22 females; mean age of 63.07 +/- 14.71 yr) were recruited. Background levels of MF were determined for each subject using a cloning assay. The mean MF/10(6) clonable cells (MF) for the study subjects was 4.63 +/- 2.20. An interview-administered questionnaire was completed by each study subject in order to assess details of dietary history, physical activity, health and potential genotoxin exposure history. A 7-day estimated dietary record method with a food frequency questionnaire was used to determine average intakes of energy and macronutrients (including alcohol), and a range of micronutrients (including vitamin and mineral supplement usage). The relationships between individual dietary and lifestyle factors and HPRT MF were investigated by univariate and multivariate analysis (data was adjusted for age, lymphocyte plating efficiency [PE] and energy intake [EI]). Univariate analysis revealed a significant positive correlation between EI and MF and multivariate analysis revealed significant positive correlations between, body mass index (BMI), % energy intake from total carbohydrate, starch, fat and MF. These findings suggest that a reduction in EI may be a useful preventative measure against the onset of carcinogenesis in humans. No correlations were found between alcohol intake and MF or between estimated antioxidant intake and MF. Thus, estimated intakes of antioxidants may not reflect their bioavailability and functional capacity in vivo and it may be more useful to examine actual plasma/cell levels vs. MF to establish if any significant relationship exists.

Levels of peripheral blood cell DNA damage in insulin dependent diabetes mellitus human subjects.

Increased production of reactive oxygen species (ROS) in vivo can lead to cellular biomolecule damage. Such damage has been suggested to contribute to the pathogenesis of insulin dependent diabetes mellitus (IDDM). In this study, we used the alkaline comet assay to measure DNA damage (single-stranded DNA breaks and alkali-labile sites) in freshly isolated whole blood, lymphocytes, monocytes, and neutrophils from 23 subjects with IDDM and 32 age- and sex-matched controls. Analysis of the results showed elevated levels of DNA damage (expressed as % comet tail DNA) in the lymphocyte (4.10+/-0. 47, 3.22+/-0.22), monocyte (4.28+/-0.47, 3.49+/-0.18), and whole blood (4.93+/-0.51, 4.51+/-0.23) fractions from IDDM subjects compared to controls, respectively, but the increases observed were not statistically significant. However, we found significantly elevated basal levels of DNA damage in the neutrophil fraction (8. 38+/-0.64, 4.07+/-0.23; p<0.001, Mann-Whitney U test) in IDDM subjects compared to controls. Given these novel neutrophil findings, we extended the study to include a total of 50 IDDM subjects and 50 age- and sex-matched control subjects and determined basal levels of DNA damage in the neutrophils of all 100 subjects. We found significantly elevated mean levels of DNA damage (8.40+/-0.83, 4. 34+/-0.27; p<0.001, Mann-Whitney U test) in the neutrophils from the IDDM subjects when compared to controls. Our results show that even with acceptable glycaemic control there is a significantly elevated level of DNA damage within diabetic neutrophils in vivo.

Mutation and the Aging Process : Mutant Frequency at the HPRT Gene Locus as a Function of Age in Humans.

Aging is a complex, biological process that is contributed to by intrinsic (genetic) and extrinsic (nutrition, infectious agents, xenobiotic exposure, etc.) factors (1). Several decades ago it was first proposed that instabilities in the organization and expression of the genetic material was likely to be involved in the aging process (reviewed in ref. 2). Indeed, since that time much experimental evidence has been published that details increases in DNA damage (3-8) and mutation (8, 9-15) in various cells and tissues with age in humans.

Food additive lactic acid production by immobilized cells of Lactobacillus brevis on delignified cellulosic material.

Improvements in yield and productivity in lactic acid fermentation by Lactobaccilus brevis cells immobilized on delignified cellulosic (DC) material are reported. The system proved to be more efficient in comparison with the work reported by other workers. Yields of 80 and 100% conversion using glucose were obtained at 30 degrees C in 1 day of fermentation time. Lactic acid fermentation using whey as substrate was obtained at 30 degrees C in 1-1.5 days, resulting in 70% yield, whereas the remaining lactose in whey was converted to alcohol byproduct, leading to a 90% lactose exploitation and 100% conversion. Cell immobilization of L. brevis on DC material was proved by its reuses in repeated batch fermentations and through electron microscopy. A series of 10 repeated batch fermentations without any loss in cell activity showed a tendency for high operational stability. The presence of DC material resulted in a drastic drop of the fermentation time from 48 to 13 h.

Lymphocyte cytochrome P450-CYP2E1 expression in human IDDM subjects.

Cytochrome P4502E1, a phase I enzyme, has been shown to be induced in liver samples from diabetic and obese rats. One study demonstrated elevated levels of CYP2E1 in children with IDDM, using Western blot analysis. The aim of this investigation was to determine CYP2E1 expression in peripheral blood lymphocytes from eight well-controlled IDDM and eight sex- and age-matched control subjects using Western blot analysis and Phoretix image analysis. Levels of CYP2E1 were low to undetectable in human lymphocytes from healthy control subjects. However, levels of CYP2E1 were elevated in lymphocytes from IDDM subjects (mean 3.1-fold higher). The elevated levels of CYP2E1 in the IDDM subjects showed no correlation with HbA(1c) nor duration of IDDM; however, there were marked inter-individual differences in levels of induction of CYP2E1 between all subjects. The results of this study suggest that in human IDDM subjects, even with good metabolic control, expression of CYP2E1 is elevated when compared to controls. CYP2E1 is known to generate ROS in vivo, the modulation of this isoform in lymphocytes from IDDM subjects, could well add to the oxidative stress associated with IDDM and the development of associated complications.

Nutrition and the ageing process.

Ageing is a complex biological phenomenon, and many of the contributing mechanisms have not yet been identified. With increasing age there is an increased risk of development of a number of age-related pathologies. In recent years experimental evidence has highlighted the role of free radical-induced biomolecule damage in the pathogenesis of such diseases. It may be that antioxidant intervention, by inhibiting or reducing free radical toxicity, could alleviate or delay the symptoms of ageing and chronic disease. In this article some of the scientific evidence in support of this is presented. In addition, other dietary factors which have been shown to be of importance for the prevention/delay of onset of age-related pathologies are discussed.

Detection of reverse transcriptase in culture medium for mammary tumour cell lines: a comparison of an established radio-labelling technique and a contemporary non-isotopic technique.

Classically, radio-label techniques have been employed to analyse biological samples for reverse transcriptase (RT) activity. More recently, however, non-isotopic kits have been developed for retroviral quantification. Nevertheless, until the present investigation it has not been known if these contemporary methods are more sensitive at detecting reverse transcriptase activity. In our study, a non-isotopic ELISA method was shown to be considerably more sensitive than the radio-label technique at detecting reverse transcriptase in growth medium used to culture the murine breast cancer cell line GR/A. Using the ELISA, less reverse transcriptase activity was demonstrated in growth medium from human mammary adenocarcinoma MCF-7 cells than the murine source. This ELISA did not detect reverse transcriptase activity from a pure source of Moloney murine leukaemia virus. In light of this, the broad applicability of this ELISA for reverse transcriptase from different viral sources must be investigated before it can be used to monitor biological supernatants for the presence of retroviruses.