Ex vivo transduction of corneal epithelial progenitor cells using a retroviral vector.

PURPOSE: To transduce corneal epithelial progenitor cells with a reporter gene using a retroviral vector and follow their progeny in vitro and in vivo. METHODS: Using a lacZ-producing retroviral vector, rabbit keratolimbal explants were transduced ex vivo, autografted onto their original sites, and assessed for lacZ-producing cells in the cornea throughout a 6-month period. Four autografts served as control samples, having received no vector. Experimental and control rabbits were euthanized and corneas with scleral rims harvested, weekly for 4 weeks and then monthly for 6 months. The corneas were first stained with 5-bromo-4-chloro-3-indolyl galactopyranoside (X-gal) as wholemounts and then sectioned for histology and immunohistochemistry to examine lacZ-positive cell outgrowth. Three additional transduced explants were observed in culture. These explants were transferred to new culture dishes every week for 9 weeks. The previously occupied culture dish was stained for lacZ to detect transduced epithelial cells, and the number of lacZ-positive cells was quantitated. RESULTS: LacZ-positive cells were found in the corneas of 18 of 20 eyes in which virally transduced keratolimbal autografts had been implanted. The cells were epithelial in nature, originated from the limbus, and were found in colonies throughout the epithelial layer of the cornea. The appearance of lacZ-positive cells in four of five corneas harvested after 6 months showed long-term transgene expression consistent with transduction of corneal epithelial stem cells. In vitro, the number of lacZ-positive cells migrating from the keratolimbal autografts decreased rapidly during the first 4 weeks and then remained stable through week 9. CONCLUSIONS: This study shows that a retroviral vector can effectively transduce corneal epithelial progenitor cells, shown by the long-term appearance of transduced cells on the cornea in vivo and the stable production of lacZ-positive cells in vitro. The appearance and disappearance of labeled cells is consistent with the initial transduction of stem cells and transient amplifying cells.

Delta 6-desaturase: improved methodology and analysis of the kinetics in a multi-enzyme system.

A new method of assay for the delta 6-desaturation of linoleic acid was developed. This method, which uses HPLC for separation of the fatty acid substrate and product, exhibited a lower coefficient of variation (0.3%) than the reported TLC method (3.5%), and avoided the step of methylation of the saponified fatty acid substrate and product. Using this new method of assay, the kinetics of the delta 6-desaturase in a multi-enzyme system were analysed. A number of factors that could have striking effects on desaturase kinetics were investigated, including the effect of (i) endogenous microsomal linoleic acid on total substrate concentration, and (ii) the pre-reaction catalysed by acyl-CoA synthetase and competing reactions catalysed by lysophospholipid acyltransferase and acyl-CoA hydrolase. Endogenous free linoleate in the hepatic microsomes was found to be 2.9 +/- 1.0 microM (0.5 mg microsomal protein/ml), which was comparable to added substrate concentrations (1.8 to 7.9 microM). The kinetics of the delta 6-desaturase were dissected from the kinetics of the above mentioned pre-reaction and competing reactions through a combination of experimental approaches and computer modeling. From computer modeling, a Km and Vmax of 1.5 microM and 0.63 nmol/min were calculated for the delta 6-desaturase, compared to Km and Vmax of 10.7 microM and 0.08 nmol/min calculated directly from data uncorrected for endogenous substrate. It was concluded that lysophospholipid acyltransferase, acyl-CoA synthetase and endogenous linoleic acid significantly affect the kinetic measurements of hepatic microsomal delta 6-desaturase. These results have implications for kinetic analyses of all desaturates in microsomal systems.

Isoflurane: a comparison of its metabolism by human and rat hepatic cytochrome P-450.

Isoflurane is metabolized by both rat and human hepatic microsomal cytochrome P-450 in vitro to fluoride ion and organofluorine metabolites. The forms of rat liver microsomal cytochrome P-450 induced by phenobarbital and pregnenolone-16 alpha-carbonitrile appear to be involved in the metabolism of isoflurane, while the forms induced by beta-naphthoflavone do not. Different pathways are favored for the metabolism of isoflurane by rat and by human liver microsomes: trifluoroacetaldehyde appears to be produced from isoflurane by rat liver microsomal cytochrome P-450, while trifluoroacetate or other nonvolatile fluorinated metabolites were not. The trifluoroacetaldehyde so produced binds tightly to microsomal constituents. Human liver microsomes converted isoflurane extensively to nonvolatile fluorinated products, one of which appears to be trifluoroacetate. The proposed pathways for the metabolism of isoflurane are considered in view of the above results.

Limitations of the sodium fusion assay for fluorinated metabolites.

A method involving sodium fusion and analysis by fluoride electrode, which has been used for the determination of total fluorinated metabolites of fluorine-containing drugs in physiologic fluids, is shown to be inapplicable to volatile fluorinated metabolites of volatile fluorinated anesthetic agents. The yields of inorganic fluoride from the volatile metabolites 2,2,2-trifluoroethanol and trifluoroacetaldehyde were less than 2% of expected values, whether these metabolites were present in water, buffer, urine, or hepatic microsomes, although reliable results were obtained for the relatively nonvolatile sodium trifluoroacetate. It is proposed that the assay does not provide a valid method for determining total fluorinated metabolites of volatile or nonvolatile fluorine-containing drugs, but may be of some use in determining a single nonvolatile fluorinated metabolite of a volatile fluorinated drug.

1,1,1-Trichloropropene-2,3-oxide: an alternate mechanism for its inhibition of cytochrome P-450.

Incubation of hepatic microsomes from phenobarbital treated rats, 1,1,1-trichloropropene-2,3-oxide (TCPO) (3 mM), EDTA (0.2 mM) and NADPH-gene-rating system in vitro decreased the levels of cytochrome P-450 by 40% and equivalently decreased microsomal heme. Appreciable losses of cytochrome P-450 were not observed [1] if metyrapone (2.3 mM) or CO:O2 (80:20; v/v) were included in incubation mixtures, [2] if the TCPO or NADPH-gene-rating system were omitted from reaction mixtures, or [3] if microsomes from untreated or beta-naphthoflavone treated rats were utilized. The time course for the TCPO mediated loss of hepatic microsomal cytochrome P-450 showed a time lag of 5 min, before the levels of cytochrome P-450 were significantly altered, followed by a 10-15 min period in which the levels of cytochrome P-450 rapidly decreased to a non-zero plateau level. The concentration of TCPO required for half maximal loss of cytochrome P-450 was ca. 0.5 mM. In the absence of cytochrome P-450 degradation, TCPO (2 mM) was an effective inhibitor of aminopyrine demethylase and benzpyrene-3-hydroxylase but not of p-nitroanisole demethylase or ethoxyresorufin deethylase. In contrast, the degradation of cytochrome P-450 by TCPO strikingly decreased ethoxyresorufin deethylase and to a lesser extent p-nitroanisole demethylase. A reaction mechanism is proposed for the TCPO mediated degradation of cytochrome P-450 in vitro.

The effect of fluroxene [(2,2,2-trifluoroethoxy)ethane] on haem biosynthesis and degradation.

Acute fluroxene treatment of male Wistar rats decreases the amounts of hepatic microsomal cytochrome P-450 and haem, increases the activities of hepatic delta-aminolaevulinate synthase and haem oxygenase, and increases the amounts of haem precursors (delta-aminolaevulinate and porphobilinogen) in the urine. All of the above effects of fluroxene are enhanced by pretreatment of the experimental animals with 3-methylcholanthrene and phenobarbital. The amounts of porphyrins in the urine and faeces were generally unaffected by acute fluroxene treatment of uninduced or 3-methylcholanthrene- or phenobarbital-induced Wistar rats. 2,2,2-Trifluoroethyl ethyl ether, the saturated analogue of fluroxene, did not affect the amounts of hepatic cytochrome P-450 and haem, the amounts of any of the haem precursors in the urine or faeces, or the activity of hepatic haem oxygenase in phenobarbital-induced male Wistar rats. The amounts of hepatic cytochrome P-450 and haem and of the haem precursors in urine and faeces, and the activity of delta-aminolaevulinate synthase, were generally not altered by acute fluroxene treatment of uninduced male Long-Evans rats. Chronic treatment of Wistar rats with fluroxene resulted in small increases in the amounts of delta-aminolaevulinate and porphyrins in urine. The amounts of porphobilinogen in urine were elevated up to 2000%, whereas the amounts of the porphyrins in faeces were generally unaffected. After chronic fluroxene treatment, the activity of delta-aminolaevulinate synthase was increased, whereas the activity of uroporphyrinogen synthase was decreased. It is concluded that acute fluroxene treatment may affect haem biosynthesis and degradation by a mechanism similar to allylisopropylacetamide, namely by stimulating an atypical cytochrome P-450-dependent pathway for haem degradation. The effects of chronic fluroxene treatment on haem biosynthesis may be a consequence of this mechanism or a result of the inhibition by fluroxene of uroporphyrinogen synthase. Chronic fluroxene treatment of male rats affects the haem biosynthetic pathway in a manner similar to that seen in human genetic acute intermittent porphyria.

Methionine sulfoxide cytochrome c.

Cytochrome c has been chemically modified by methylene blue mediated photooxidation. It is established that the methionine residues of the protein have been specifically converted to methionine sulfoxide residues. No oxidation of any other amino acid residues or the cysteine thioether bridges of the molecule occurs during the photooxidation reaction. The absorbance spectrum of methionine sulfoxide ferricytochrome c at neutrality is similar to that of the unmodified protein except for an increase in the extinction coefficient of the Soret absorbance band and for the complete loss of the ligand sensitive 695 nm absorbance band in the spectrum of the derivative. The protein remains in the low spin configuration which implies the retention of two strong field ligands. Spin state sensitive spectral titrations and model studies of heme peptides indicate that the sixth ligand is definitely not provided by a lysine residue but may be methionine-80 sulfoxide coordinated via its sulfur atom. Circular dichroism spectra indicate that the heme crevice of methionine sulfoxide ferri- and ferrocytochrome c is weakened relative to native cytochrome c. The redox potential of methionine sulfoxide cytochrome c is 184 mV which is markedly diminished from the 260 mV redox potential of native cytochrome c. The modified protein is equivalent to native cytochrome c as a substrate for cytochrome oxidase and is not autoxidizable at neutral pH but is virtually inactive with succinate-cytochrome c reductase. These results indicate that the major role of the methionine-80 in cytochrome c is to preserve a closed hydrophobic heme crevice which is essential for the maintainance of the necessary redox potential.

An investigation into the hepatic cytochrome P-450 catalysed metabolism of the anaesthetic fluroxene (2,2,2-trifluoroethyl vinyl ether).

The role of the different cytochromes P-450 in the metabolism of the anaesthetic agent fluroxene, and the mechanism of production of toxic effects seen after pre-treatment of the animals with pehnobarbital prior to anaesthesia, have been investigated. Male rats were anaesthetized with fluroxene, or with 2,2,2-trifluroethyl ethyl ether, or with ethyl vinyl ether in an attempt to ascertain the in vivo toxic effects of the three anaesthetic agents. The resultant hepatic histology is reported. A study of the binding and metabolism of fluroxene by isolated rat hepatic microsomes was also made. We conclude that it is elevated levels of cytochrome P-450 which potentiate the toxicity of fluroxene anaesthesia in phenobarbital treated animals and that cytochrome P-448 does not bind or metabolize fluroxene. The potential toxicity of the fluroxene molecule is considered to reside in the trifluoroethyl moiety, while the vinyl group of fluroxene appears to play a role in the observed liver damage.