Coexistence effect of UVA absorbers to increase their solubility and stability of supersaturation

OBJECTIVE

Sunscreens containing UVA absorbers in high concentrations are expected to be developed, since recent studies have suggested the possibility of involvement of UVA ray in skin cancer and early skin aging. Solubility and stability of supersaturation of UVA absorbers in UVB absorber were determined in the absence and the presence of cosmetic oil. Coexistence effect of UVA absorbers was analyzed to dissolve them in high concentrations.

METHODS

Two UVA absorbers, diethylamino hydroxybenzoyl hexyl benzoate (DHHB) and butyl methoxydibenzoylmethane (BMDM), a UVB absorber, 2-ethylhexyl methoxycinnamate (EHMC), and a cosmetic oil, 2-ethylhexyl ester of oligomer of hydroxystearic acid (EH-O-HSA), were used. Their solutions were prepared at 80°C and cooled to 5°C. The solid DHHB and/or BMDM were added to it, and the time evolution of concentrations of the UVA absorbers in the solution phase was monitored.

RESULTS

At the saturation in the absence of EH-O-HSA at 5°C, weight ratio of DHHB and BMDM to EHMC was 0.39/1.00 and 0.22/1.00, respectively. Addition of EH-O-HSA slightly changed the solubility of DHHB and BMDM. When the weight ratio of EH-O-HSA to EHMC was 0.20/1.00, weight ratio of DHHB and BMDM to EHMC was 0.35/1.00 and 0.25/1.00, respectively at the saturation at 5°C. In the presence of EH-O-HSA, a strong coexistence effect of DHHB and BMDM was found on their solubility. A thermodynamically stable saturated solution at 5°C having the composition that DHHB: BMDM: EHMC: EH-O-HSA = 0.47: 0.46: 1.00: 0.20 was obtained by the simultaneous addition of solid DHHB and BMDM into the initial solution.

CONCLUSION

The solution type composite having the highest concentrations of DHHB and BMDM prepared in this study exhibited critical wavelength at 368 nm that was just below the border for sunscreens being qualified as 'Broad Spectrum' protection under the new rule launched by US FDA.

Positron annihilation in hydrogen-doped alkali-metal fullerides

Hydrogen-doped alkali-metal fullerides were studied by means of mass-analyzed thermal desorption and positron annihilation. The hydrogen desorption spectra reveal the inclusion of hydrogen in the fullerides as two types of components; one weakly adsorbed and the other strongly bound, probably as hydride ion. The positron lifetime spectra demonstrate that hydrogen-doping to alkali-metal fullerides increases a concentration of higher-order vacancy-type defects, through which more than 50% injected positrons decay with a lifetime of 0.42–0.71 ns.

Abortion in a thoroughbred mare associated with an infection with avirulent Rhodococcus equi

An eight-year-old thoroughbred mare with no previous history of illness aborted a fetus at 196 days of gestation, and its internal tissues were examined immunohistologically and bacteriologically. The placenta was not examined, but specimens of the intrauterine fluids and the dam's faeces were collected four days after the abortion and examined bacteriologically. No significant histological lesions were found in the fetus but the amnion and the umbilical cord were oedematous and had petechial haemorrhages. Rhodococcus equi was isolated in pure culture from the lung, heart and stomach contents of the fetus and from an intrauterine specimen and faeces of the dam. The anti-R equi antibody titre of the mare was high after the abortion. The diagnosis was confirmed in the lung of the fetus by immunohistochemical staining with R equi-specific antibodies. Isolates from the fetus and mare were identified as avirulent R equi by pcr and the mouse pathogenicity test. The avirulent isolates were characterised by pulsed-field gel electrophoresis, which yielded only one VspI profile in all the isolates from the fetus and its dam.

Current-free double-layer formation in inductively coupled plasma in a uniform magnetic field

The axial profiles of plasma parameters for low and moderate pressures, such as the plasma potential, electron temperature, and number density, have been evaluated in magnetized inductively coupled plasma. The experimental results revealed in both cases the existence of a genuine current-free double-layer structure, separating two plasma regions with different properties. Based on the experimental results, a physical scenario for the self-assembling of the double layer is proposed. Also, the axial profile of the electron number density downstream is analyzed, emphasizing the role of neutral metastable ionization, and a simple analytical model is developed to fit the experimental data. The model allows the estimation of neutral metastable number density downstream and the recombination rate coefficient.

Interrupted aortic arch with ventricular septal defect: an unsuspected fatal congenital anomaly in forensic pathology

Interrupted aortic arch (IAA) is a relatively rare cardiovascular anomaly worldwide and is very rarely encountered in forensic pathology. A case of clinically unsuspected fatal IAA in a 5-day old neonate is described. A full term female baby was declared dead on arrival at an emergency hospital few hours after having been discharged from a maternity hospital following delivery. There were no noticeable signs and symptoms reported during the short postnatal period. Antenatal medical history was unremarkable and there were no abnormalities noticed at birth. Autopsy revealed an unsuspected IAA arch with ventricular septal defect (VSD) as cause of sudden unexpected death. Although amenable to surgical intervention, IAA may not produce obvious clinical symptoms as was the case for this baby. Such sudden unexpected deaths raise medico-legal questions as to the manner and the precise cause of death. The importance of a full forensic autopsy to clarify such a sudden death was re-emphasized.

Complete nucleotide sequences of genome segments 1 and 3 of Rosellinia anti-rot virus in the family Reoviridae

The nucleotide sequences of genome segments 1 and 3 of Rosellinia anti-rot virus (RArV) from a hypovirulent isolate, W370, of the plant pathogen Rosellinia necatrix were determined. The complete nucleotide sequence of the genome segment 1 encoded a putative RNA-dependent RNA polymerase (RDRP). The deduced amino acid sequence of RDRP of RArV showed 29% identity with RDRPs of Colorado tick fever virus (CTFV) and European Eyach virus (EYAV) in the genus Coltivirus, and identities of 23-21% with members of the genera Fijivirus and Cypovirus. Both RArV and the Coltivirus member might have originated from a common virus ancestor.

Molecular evolution of human echovirus 9 isolated from patients with aseptic meningitis in northern Kyushu during the summer of 1997

An epidemic of aseptic meningitis caused by human echovirus 9 (E-9) occurred in the summer of 1997 in northern Kyushu, Japan. Sequences of genome position 2504-3358, which encoded a part of VP1, of the nine isolated viruses were determined. An RGD motif and B-C loop were found in all. They were almost identical and closely related to the virulent strain Barty.

Preparation strategy for uniformly sized, polymer-based HPLC packing materials having practically acceptable column efficiency. 1. Copolymerization technique

Uniformly sized, polymer-based packing materials affording excellent chromatographic performance in semimicro HPLC were prepared from mixtures of alkyl methacrylate and glycerol dimethacrylate by a copolymerization technique using a multistep swelling and polymerization method. A column efficiency of up to 13000 plates was obtained for a 2-mm-i.d. x 150-mm column with beads prepared from a 1:1 (v:v) mixture of an alkyl methacrylate and glycerol dimethacrylate. The packing materials exhibit a considerable improvement in column efficiency, as demonstrated on the separation of polyaromatic hydrocarbons (PAHs) having a relatively long retention time, which thus far has been a serious problem of polymer-based packing materials. Detailed studies suggested a rather limited optimum pore size, and its distribution was depicted to get the excellent column efficiency on polymer-based packing materials.

Unusual homicide by air gun with pellet embolisation

A homicidal case by an air gun involving a 73-year-old woman who was found dead in her home is described. At autopsy, two gunshot wounds were observed on the left anterior chest and the right back. There was neither exit wound nor powder burns. The fatal pellet, 0.9 g and 5.5mm diameter, penetrated the left anterior chest and the heart to cause cardiac tamponade (200 ml) before lodging in the left subclavian artery. Death was due to cardiac tamponade. This case has exhibited two rare phenomena: homicide by an air gun and pellet embolisation. This is the first homicide case by use of air gun in Japan and demonstrates that although it may be rare, air guns can be used as potential homicidal weapons. This experience will alert our society and others on the potential hazards of air guns we have tended to regard as toys.

Isolation of amantadine-resistant influenza a viruses (H3N2) from patients following administration of amantadine in Japan

In Japan, the use of amantadine for treatment of influenza A virus infection was not accepted until November 1998, although it was widely used for treatment of Parkinsonism. Since then, we have monitored the emergence of amantadine-resistant viruses and isolated two viruses from patients on long-term treatment with amantadine.

Influenza virus RNA polymerase PA subunit is a novel serine protease with Ser624 at the active site

BACKGROUND

Influenza virus RNA polymerase is a multifunctional enzyme that catalyses both transcription and replication of the RNA genome. The function of the influenza virus RNA polymerase PA subunit in viral replication is poorly understood, although the enzyme is known to be required for cRNA --> vRNA synthesis. The protease related activity of PA has been discussed ever since protease-inducing activity was demonstrated in transfection experiments.

RESULTS

PA protein was highly purified from insect cells infected with the recombinant baculovirus carrying PA cDNA, and a novel chymotrypsin-type serine protease activity was identified with the synthetic peptide, Suc-LLVY-MCA, in the PA protein. [3H]DFP was crosslinked with PA and a mutational analysis revealed that serine624 was as an active site for the protease activity.

CONCLUSIONS

These results constitute the demonstration of protease activity in PA subunit of the influenza virus RNA polymerase complexes.

Antibody-scanning and epitope-tagging methods; molecular mapping of proteins using antibodies

Because synthetic short peptides bearing critical binding residues, can chemically mimic the folded antigenic determinants on proteins, short synthetic peptides can generate antibodies that react with cognate sequences in intact folded proteins. According to this mimotope theory, we produced site-specific antibodies by immunization with short peptides which overlapped each other and covered the entire protein, and used them for domain mapping of influenza virus RNA polymerase (antibody-scanning method). We also used a tagged-epitope and its monoclonal antibodies for topology mapping of clathrin light chains in clathrin triskelions by electron microscopy. Both methods using specific epitopes in combination with their antibodies enable us to determine the domains of interesting proteins systematically without the need to generate monoclonal antibodies or mutant proteins.

Inhibition of influenza virus replication in cultured cells by RNA-cleaving DNA enzyme

Influenza virus replication has been effectively inhibited by antisense phosphothioate oligonucleotides targeting the AUG initiation codon of PB2 mRNA. We designed RNA-cleaving DNA enzymes from 10-23 catalytic motif to target PB2-AUG initiation codon and measured their RNA-cleaving activity in vitro. Although the RNA-cleaving activity was not optimal under physiological conditions, DNA enzymes inhibited viral replication in cultured cells more effectively than antisense phosphothioate oligonucleotides. Our data indicated that DNA enzymes could be useful for the control of viral infection.

Japanese monozygotic twins with familial amyloidotic polyneuropathy (FAP) (ATTR Val30Met)

Twenty-nine-year-old twin brothers having the amyloidogenic transthyretin (ATTR) Val30Met gene developed the clinical symptoms of familial amyloidotic polyneuropathy (FAP) in 1995. The twins had the same educational background and lived in the same district. FAP manifestations were similar in both cases, although electromyographic examinations revealed sensorimotor polyneuropathy in No. 1 and sensory polyneuropathy in No. 2. DNA analysis revealed that they were monozygotic twins. In addition to environmental factors, genetic factors may play an important role in determining the onset of FAP.

Perinatal outcome and management of single fetal death in twin pregnancy: a case series and review

OBJECTIVE

To determine neonatal outcome of single fetal death in twin pregnancy.

METHODS

Retrospective study was performed on 30 cases of single fetal death in twin pregnancy during the period from August 1971 to December 1997 at Kitasato university hospital.

RESULTS

The incidence of single fetal death in twin pregnancy was 6.2% among total number of 481 twins. Prognosis of surviving infants was satisfactory except in one case, with malformation in 10 cases of dichorionic twins (total 242 sets). In contrast, neonatal death occurred in 6 cases of 20 monochorionic twins (total 239 sets), 4 of which were among 7 cases of twin-to-twin transfusion syndrome (TTTS). Cerebral palsy occurred in 2 infants of monochorionic twins. One case was diagnosed as periventricular leukomalacia (PVL) within one week after birth, and this was the case of delivery 1 day after one twin fetal death.

CONCLUSION

Single fetal death itself occurs more often in monochorionic twins. There seems to be a benign outcome for the surviving dichorionic twins. On the other hand, there may be a grave outcome, though infrequent, for monochorionic twins when concomitant TTTS has existed. It is very important to identify the chorionicity by ultrasound examination in early pregnancy.

Death as a result of heat stroke in a vehicle: an adult case in winter confirmed with reconstruction and animal experiments

A 54-year-old man was found dead in the driver's seat of his vehicle on a winter's day. Investigations of the vehicle revealed that the engine was running, and the car heater was left on with the maximum temperature and velocity. The body was found excessively sweating. Rectal temperature of the body was 43 degrees C at 10 h post mortem. In autopsy, several superficial skin burns were observed on the face, the shoulders and the legs. The lungs were heavily congested and hemorrhagic. The liver showed typical alcohol-induced micronodular cirrhosis. The alcohol concentrations were 0.17% in the blood of both the left and the right heart, 0.17% in the femoral-vein blood, 0.21% in the bladder urine and 0.34% in the gastric contents. A reconstruction experiment demonstrated that the temperature inside the vehicle rose rapidly and reached 50-58 degrees C in 3 h. Animal experiments showed that the temperature threshold for rats to succumb to heat was between 40 and 45 degrees C. This case shows that heat stroke in a vehicle can occur in adults with chronic diseases or alcoholism, such as in this particular case, even in the winter.

Nucleotide sequence of seed- and pollen-transmitted double-stranded RNA, which encodes a putative RNA-dependent RNA polymerase, detected from Japanese pear

The nucleotide sequence of the largest double-stranded (ds) RNA (named dsRNA1) of three species of seed- and pollen-transmitted dsRNA species detected from Japanese pear was analyzed, and one strand was found to contain a single long open reading frame (ORF) of 1434 nucleotides that encoded a putative polypeptide containing 477 amino acid residues with a molecular mass of 54.9 kDa. This polypeptide contained amino acid sequence motifs conserved in putative RNA-dependent RNA polymerases of RNA viruses. Attempts to visually identify or purify virus-like particles associated with the dsRNAs were unsuccessful. Slow sedimentation of the dsRNA fraction suggests that the dsRNAs may be unencapsidated. The concentration of dsRNAs in the host, Japanese pear, was about 16 times higher than that from a cryptic virus, radish yellow edge virus (RYEV). These results suggest that the dsRNAs were not from cryptic viruses. Partial nucleotide sequences of the two smaller dsRNAs (named dsRNAs 2 and 3) and two other dsRNAs (named dsRNAs 4 and 5) detected from only the Japanese pear cultivar (cv.) Akita Tazawa 3 Gou were analyzed, and encoded nearly the same amino acid sequence encoded by dsRNA1.

Nucleotide sequence of the gene encoding the RNA polymerase and the 3' non-coding region of a bovine enterovirus Japanese isolate: rapid synonymous substitutions between European and Japanese strains

The nucleotide sequences of the genome RNA encoding the RNA polymerase and the 3' non-coding region (NCR) of bovine enterovirus (BEV) serotype I Japanese isolate, MZ468, were determined. The genetic distance between the two BEV serotype I strains, MZ468 and VG-5-27, was calculated by pairwise comparison of nucleotide sequences. The synonymous substitution rate was high (1.40 x 10(-2)/site/year), and of the same order as those of influenza virus HA, HIV-1 gag and env, and enterovirus 70 VP1 genes.

Analysis of ibuprofen metabolites by semi-microcolumn liquid chromatography with ultraviolet absorption and pulsed amperometric detectors

Semi-microcolumn (1.5 mm i.d.) liquid chromatography (LC) system with ultraviolet (UV) and pulsed amperometric detector (PAD) was constructed for analysis of ibuprofen metabolites in human urine. PAD was connected in series with the UV detector, and an alkaline solution was post-column added to a mobile phase after the UV detector. By a gradient elution, five ibuprofen metabolites were detected with UV detection from 1 microliter of human urine at 3.5 h after the administration, and information concerning their glucuronation was simultaneously obtained by PAD response.

Synthesis and characterization of polymer-coated mixed-functional stationary phases with several different hydrophobic groups for direct analysis of biological samples by liquid chromatography

Three types of polymer-coated mixed-functional (PCMF) silica column-packing materials for the direct analysis of biological liquids have been synthesized. These packing materials all have polyoxyethylene groups as their hydrophilic part but differ in their hydrophobic part (methyl groups for Me-POE, phenyl groups for Ph-POE, and octyl groups for Oc-POE). Retention characteristics with respect to several drug molecules, and protein recovery, were studied for each of the three types with different amounts of hydrophobic groups attached. Of the PCMFs prepared, Oc-POE showed the greatest overall retention, and Me-POE displayed the greatest protein recovery.

Synthesis of polymer-coated mixed-functional packing materials for direct analysis of drug-containing serum and plasma by high-performance liquid chromatography

Silicone polymer-coated mixed-functional (PCMF) silica packing materials were developed for the direct determination of drugs contained in serum or plasma. The new stationary phases were prepared by the following three-step procedure: (1) coating porous silica with a silicone polymer; (2) partial introduction of hydrophobic groups; and (3) introduction of hydrophilic groups. Two PCMFs were synthesized, one having polyoxyethylene groups as hydrophilic groups (PCMF-POE) and the other having oligoglyceryl groups (PCMF-OG). PCMF-POE showed higher recoveries for injected proteins and a greater overall retention for drug molecules than PCMF-OG. PCMF-POE did not show any column deterioration in 500 serum sample injections (10 ml in total).

Giant extrathoracic hematoma after thoracoplasty--a case report

A rare case of an extrathoracic giant hematoma developing after a thoracoplasty in a 67 year old man is presented herein. The patient underwent a right thoracoplasty without removal of plombage and a left thoracoplasty with removal of plombage for tuberculosis of the bilateral upper lobes 27 and 24 years prior to presentation, respectively. He presented to us in May, 1987, with a subscapular tumor which had been growing over the last 5 years. A giant tumor, measuring 23 x 17 x 12 cm and weighing 2585 g was successfully removed and the patient has since been well without any evidence of recurrence.

Densitometric determination of human papillomavirus DNA quantities by chromato-scanning in the fluorescence mode

The quantitation of human papillomavirus DNA isolated from warts by chromato-scanning (fluorescence mode) photographs of ethidium bromide-stained agarose gels is described. Excitation at 200 nm (with a cutoff filter at 400 nm) generates fluorescence from the white portion of the printing paper. The fluorescent intensity correlated with the quantities of DNA in the band of interest. The amounts of DNA were determined using calibration curves of approximately the same size as lambda phage DNA fragments. This general method of quantification is applicable to photographs of other types of polynucleotides capable of being separated and stained in a gel medium.

Performance of wide-pore silica- and polymer-based packing materials in polypeptide separation: effect of pore size and alkyl chain length

The effects of pore size and alkyl chain length of silica- and polymer-based packing materials in the elution of polypeptides with an acetonitrile gradient in the presence of trifluoroacetic acid were studied. Considerable differences were found in the performance of alkylsilylated phases prepared from various wide-pore silica particles assumed to have 30-50-nm pores. The pore size of such silica gels was found to be the critical factor in determining the efficiency for high-molecular-weight polypeptides. Silica C18 phases having small pore volumes below 20 nm pore diameter showed comparable performances to C4 and C8 phases for polypeptides with molecular weights of up to 80,000, and were more stable. Polymer-based packing materials with adequate pore size provided excellent column efficiencies and recoveries for polypeptides with higher chemical stabilities than silica-based materials.

Subcloning and trial of expression of the protein kinase catalytic domain of RSV-src gene

The gene of catalytic domain of the protein kinase of RSV-scr was cloned into the BamHI cloning site of translation vector pET-8c which containing T7 RNA polymerase promotor, and transformed BL21 (DE3) pLys S (Studier and Moffatt, 1986). The putative molecular weight of the protein was about 33 kd as evaluated on the basis of its nucleotide size showed the identical mobility in SDS-polyacrylamide gel electrophoresis. However, yield of protein production was not high, probably, because of its instability in Escherichia coli.

Effects of protein phosphorylation with p43v-abl and protein kinase A catalytic subunit on fluorescence intensity

To study the effect of phosphorylation on protein conformation, a fluorescence spectroscopic study was performed on phosphorylated enolase and histoneH1 proteins. The peak of fluorescence was 330 and 360 nm for each protein, respectively, when excited at 287 nm. The intensities of the fluorescence were measured during the phosphorylation reactions with the protein kinase A and p43v-abl, for serine, threonine and tyrosine, respectively. Slightly increased intensities at 330 and 360 nm for enolase and histoneH1 protein were observed by phosphorylation with p43v-abl, whereas decreased intensities occurred with the protein kinase A catalytic subunit. These data suggest that micro-structural changes are induced at the residue, either tyrosine, serine or threonine in the protein.

Elevation of sialyl stage-specific mouse embryonic antigen levels in pleural effusion in patients with adenocarcinoma of the lung

Sialyl stage-specific mouse embryonic antigen (SSEA-1) levels were measured in pleural effusions obtained from patients with lung cancer and benign pulmonary disease, using a solid-phase immunoradiometric sandwich assay. The mean (+/- SEM) levels (unit/ml) of pleural fluid sialyl SSEA-1 were 3620 +/- 1419 in adenocarcinoma (n = 25), 123 +/- 30 in nonadenocarcinoma (n = 13) and 95 +/- 19 in benign pulmonary disease (n = 13), respectively. The positive rate was 64% in adenocarcinoma, 7.7% in nonadenocarcinoma, and 0% in benign pulmonary disease, respectively, when a cutoff level was defined as the mean + 3 SD value (300 unit/ml) based on pleural fluid sialyl SSEA-1 levels in benign pulmonary disease. There was a significant positive correlation between pleural fluid levels of sialyl SSEA-1 and those of carcinoembryonic antigen in adenocarcinoma patients (r = 0.8246, P less than 0.01). Pleural fluid sialyl SSEA-1 levels correlated with cytologic findings in adenocarcinoma patients. These observations suggest that sialyl SSEA-1 in pleural effusion is a useful marker to discriminate malignant from nonmalignant and adenocarcinoma from nonadenocarcinoma of the lung.

Selective inhibition of adenosine triphosphatase from plasma membranes of cultured endothelial cells by calcium antagonists

A study has been made of the inhibitory effect of various kinds of hypotensive agents on ATPase of cultured endothelial cells from bovine carotid artery stimulated with Ca2+. The ATPase activity was selectively inhibited by calcium antagonists including a new compound, 6-ethoxycarbonyl-5,7-dimethyl-1-[3-(4-phenethyl-1-piperazinyl)propyl]-3- (alpha, alpha, alpha-trifluoro-o-tolyl)-2,4(1H,3H)-quinazolinedione dihydrochloride, while not by other hypotensive agents.

Ca2+- or Mg2+-dependent ATPase in plasma membrane of cultured endothelial cells from bovine carotid artery

ATPase was found in plasma membrane of cultured endothelial cells from bovine carotid artery. The activity of the enzyme solubilized by octaethyleneglycol mono-n-dodecyl ether was enhanced by the addition of Ca2+ or Mg2+ and was not affected by F-actin and ouabain. Vmax was 2.8 and 10.0 mumol Pi/mg protein per h for Ca2+- and Mg2+-dependent activity, respectively, and the corresponding Km was 4.8 X 10(-4) M and 3.2 X 10(-4) M. Molecular weight of the protein was estimated to be approx. 250 000, as determined by activity-staining electrophoresis with polyacrylamide gels.

Effect of concanavalin A on the activity of membrane-bound and detergent-solubilized Mg2+ -ATPase

Plasma membranes were isolated after binding liver and hepatoma cells to polylysine-coated polyacrylamide beads, and the effect of concanavalin A on the membrane-bound Mg2+ -ATPase and the Mg2+ -ATPase solubilized by octaethylene glycol monododecyl ether (C12E8) was studied. In the experiment of membrane-bound Mg2+ -ATPase, plasma membranes were pretreated with Concanavalin A and the activity was assayed. Concanavalin A stimulated the activity of both liver and hepatoma enzymes assayed above 20 degrees C. Concanavalin A abolished the negative temperature dependency characteristic of liver plasma membrane Mg2+ -ATPase. On the other hand, Concanavalin A prevented the rapid inactivation due to storage at -20 degrees C, which was characteristic of hepatoma plasma membrane Mg2+ -ATPase. With solubilized Mg2+ -ATPase from liver plasma membranes, the negative temperature dependency was not observed. Concanavalin A, which was added to the assay medium, stimulated the activity of the enzyme solubilized in C12E8 at a high ionic strength. However, Concanavalin A failed to show any effect on the enzyme solubilized in C12E8 at a low ionic strength. With solubilized Mg2+ -ATPase from hepatoma plasma membranes, Concanavalin A could not prevent the inactivation of the enzyme during incubation at -20 degrees C.