Photooxidation of lens proteins with xanthurenic acid: a putative chromophore for cataractogenesis

The tryptophan metabolite, xanthurenic acid (Xan), is produced through a transamination reaction in high concentrations in human lenses with age and has been isolated from aged human cataractous lenses. It has appreciable absorption between 300 and 400 nm (lambda max = 334 nm), the range absorbed by the human lens. Our recent studies have shown that unlike most tryptophan metabolites in the eye, Xan is photochemically active, producing both superoxide and singlet oxygen. To determine if Xan could act as a photosensitizer and photooxidize cytosolic lens proteins, alpha-, beta- and gamma-crystallins were irradiated (lambda > 300 nm, 12 mW/cm2) in the presence and absence of Xan. Upon irradiation and in the presence of Xan, lens proteins polymerized in the order alpha > beta > gamma as assessed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Further analysis of the photolyzed alpha-crystallin by mass spectrometry indicated that histidine, tryptophan and methionine residues were oxidized at specific positions in a dose-dependent (irradiation time) manner. In alpha A-crystallin two forms of oxidized histidine 154 were observed, 2-imidazolone and 2-oxohistidine. Our results suggest that naturally occurring Xan is a chromophore capable of photosensitization and photooxidation of lens proteins. Furthermore, this compound could play a role in age-related cataractogenesis.

Identification of peptide oxidation by tandem mass spectrometry

Oxidative reactions play important roles in a variety of biochemical events ranging from normal metabolism to aging and disease processes. Proteins represent major targets for modification in these reactions, and identification of sites and structures of modifications may lead to mechanistic understanding and approaches for prevention. In this Account, the utility of mass spectrometry and its advantages are described for the identification of oxidative modifications to protein targets. A variety of examples are provided to illustrate how modifications are accurately identified and quantitated using modern methods of ionization coupled with HPLC and tandem mass spectrometry.

Identification of tryptophan oxidation products in bovine alpha-crystallin

Oxidation is known to affect the structure, activity, and rate of degradation of proteins, and is believed to contribute to a variety of pathological conditions. Metal-catalyzed oxidation (MCO) is a primary oxidizing system in many cell types. In this study, the oxidative effects of a MCO system (the Fenton reaction) on the structure of the tryptophan residues of alpha-crystallin were determined. Tandem mass spectrometry (MS/MS) was utilized to identify specific tryptophan and methionine oxidation products in the bovine alpha-crystallin sequence. After oxidative exposure, alpha-crystallin was digested with trypsin, and the resulting peptides were fractionated by reverse-phase HPLC. Structural analysis by mass spectrometry revealed that tryptophan 9 of alphaA- and tryptophan 60 of alphaB-crystallin were each converted into hydroxytryptophans (HTRP), N-formylkynurenine (NFK), and kynurenine (KYN). However, only HTRP and KYN formation were detected at residue 9 of alphaB-crystallin. Oxidation of methionine 1 of alphaA- and methionine 1 and 68 of alphaB-crystallin was also detected. The products NFK and KYN are of particular importance in the lens, as they themselves are photosensitizers that can generate reactive oxygen species (ROS) upon UV light absorption. The unambiguous identification of HTRP, NFK, and KYN in intact alpha-crystallin represents the first structural proof of the formation of these products in an intact protein, and provides a basis for detailed structural analysis of oxidized proteins generated in numerous pathological conditions.

Radiolysis-induced oxidation of bovine alpha-crystallin

Radiolysis of water by ionizing radiation results in the production of pure hydroxyl radicals. This technique, combined with analysis by tandem mass spectrometry (MS/MS), has been used to study the effect of hydroxyl radicals on the intact bovine alpha-crystallin protein. After exposure to gamma-irradiation, the oxidized alpha-crystallin was digested with trypsin and the resulting peptides were fractionated by reverse-phase HPLC. The isolated fractions were analyzed by matrix-assisted laser desorption ionization and by MS/MS to determine the locations and identities of the modifications. Structural analysis revealed that methionine 1 of alpha A- and alpha B-crystallin and methionine 68 of alpha B-crystallin were oxidized to methionine sulfoxide. Hydroxytryptophan was formed from each tryptophan residue in alpha-crystallin, although only tryptophan 9 of alpha A-crystallin was converted into N-formylkynurenine. This study has, for the first time, identified the sites of modification and the structures produced in the intact alpha-crystallin protein by exposure to hydroxyl radicals. By determining the consequences of in vitro exposure of alpha-crystallin to pure hydroxyl radicals, the in vivo contribution of this reactive oxygen species to the overall oxidative stress of the lens will be achieved from the identification of the modifications to alpha-crystallin purified from intact human lenses.

Effects of propranolol on phosphatidate phosphohydrolase and mitogen-activated protein kinase activities in A7r5 vascular smooth muscle cells

High doses of propranolol inhibit phosphatidate phosphohydrolase (PAP) activity in intact cells, thus blocking metabolism of phosphatidic acid (PA), product of the phospholipase D (PLD) reaction. Vasopressin and phorbol ester activate PLD and ERK (extracellular signal-regulated protein kinase) mitogen-activated protein kinases in A7r5, a rat vascular smooth muscle cell line. Propranolol increased PA levels in intact A7r5 cells and inhibited cytosolic PAP and membrane calcium-independent phospholipase A2 but did not activate PLD or enhance agonist-induced PA accumulation. Incubation of cells with 200 microM propranolol for 10-45 min markedly elevated PA but caused only partial activation of ERKs. Propranolol and other lipophilic amines caused a time- and dose-dependent detachment of cells from their substrate. These results confirm that elevation of PA is not a strong signal for ERK activation and emphasize that caution should be exercised in using propranolol as a PAP inhibitor in intact cells.

Identification of photooxidation sites in bovine alpha-crystallin

Because UV irradiation of proteins can produce reactive oxygen species and exposure to UV light has been implicated in cataractogenesis, the sites of photooxidation of bovine alpha-crystallin, a major lens protein with molecular chaperone activity, were identified using tandem mass spectrometry (MS/MS). Bovine alpha-crystallin was irradiated with UV light (> 293 nm) for 1, 4 and 8 h, digested with trypsin and analyzed by matrix-assisted laser desorption ionization, time-of-flight mass spectrometry (MALDI) to identify the oxidized sequences. Tryptic peptides were purified by reverse-phase HPLC and oxidized peptides were sequenced by MS/MS to determine the sites of oxidation. Tryptophan fluorescence decreased exponentially with increasing time of UV exposure and peptides containing residues 1-11 of alpha A-crystallin and 1-11, 12-22 and 57-69 of alpha B-crystallin were determined to be oxidized by shifts of 16 D or multiples of 16 Da above the mass of the unmodified peptide. The MALDI analysis revealed single oxidation of all four sequences, which increased with increasing time of UV exposure and possible double oxidation of alpha B 12-22. The specific sites of photooxidation indicate that the N-terminal regions of alpha A- and alpha B-crystallin are exposed to an aqueous environment and are in the vicinity of tryptophan residues from neighboring subunits.

Tyrosine phosphorylation of phosphatidylinositol 3-kinase and of the thromboxane A2 (TXA2) receptor by the TXA2 mimetic I-BOP in A7r5 cells

Thromboxane A2 (TXA2) interacts with its G-protein coupled receptor, the TP receptor, to produce contraction and proliferation of vascular smooth muscle cells. We have shown previously that proliferation of primary cultures of vascular smooth muscle cells initiated by [1S-(1alpha, 2beta(5Z), 3alpha(1E, 3R), 4alpha]-7-[3-(3-hydroxy-4-(4'-iodophenoxy)-1-butenyl)-7-oxab icyclo-[2.2.1]heptan-2yl]-5'-heptenoic acid (I-BOP), a stable TXA2 mimetic, is mediated by activation of mitogen-activated protein (MAP) kinase. In the present study, we examined further the intracellular mediators involved in TXA2 activation of vascular smooth muscle cells. Transient transfection of the cDNA for the TP receptor into A7r5 vascular smooth muscle cells resulted in expression of TP receptors with a receptor density, Bmax, of 0.7 +/- 0.2 pmol/mg protein and a receptor affinity, Kd, of 0.6 +/- 0.1 nM (N = 7). Mock transfected cells lacked significant receptor expression. In TP receptor transfected cells, I-BOP increased the activation of MAP kinase 2-fold, stimulated tyrosine phosphorylation of cellular proteins of relative molecular mass (Mr) of 140, 85, 60, 56, and 45 kDa, and increased the message for c-jun, a nuclear transcription factor involved in mitogenesis, 2.6-fold. Immunoblot analysis indicated that the 85-kDa protein represented phosphoinositide 3-kinase (PI3-K), while the 60 kDa protein was the TP receptor. The activity of PI3-K was increased 3.5-fold by the addition of I-BOP (0.1 microM). In summary, the present study demonstrated that stimulation of the TP receptor results in tyrosine phosphorylation of the receptor and of PI3-K.

A transforming growth factor-alpha pathway is expressed in GH4C1 rat pituitary tumors and appears necessary for tumor formation

Transforming growth factor-alpha (TGF alpha) is a growth regulatory peptide expressed largely as a high mol wt species in the anterior pituitary gland. The overall objective of this work was to test the hypothesis that altered expression of TGF alpha may play a role in the tumorigenicity of the GH4C1 cell line. We examined expression of TGF alpha in three related clones of pituitary tumor cells (GH1, GH3, and GH4C1) grown as transplantable tumors, the MtT/W5 tumor from which they were derived, and anterior pituitary glands of Wistar-Furth rats, the source of the MtT/W5 tumor. Wistar-Furth anterior pituitary, MtT/W5, GH1, GH3, and GH4C1 extracts all contained TGF alpha-specific immunoreactivity, which, when examined on sodium dodecyl sulfate-gel transfers, was of high relative mol wt, corresponding to incompletely processed TGF alpha. In neither the anterior pituitary nor the tumors was the fully processed 6-kilodalton TGF alpha form identified, indicating that mature TGF alpha is expressed to only a limited degree in normal and tumor pituitary tissue. We next determined whether a lack of receptors for TGF alpha may account for the MtT/W5 tumor phenotype in vivo. Scatchard analysis of [125I] epidermal growth factor ([125I]EGF) saturation isotherm binding identified a comparable class of sites in both the anterior pituitary gland and GH4C1 transplantable tumors. Specific binding sites were also found in MtT/W5, GH1, and GH3 tumors. Thus, the functional components of a TGF alpha pathway exist in both the anterior pituitary gland and GH4C1 transplantable tumors. We lastly examined whether a TGF alpha pathway plays a functional role in GH4C1 tumor formation. Toward this aim, we isolated TGF alpha-nonresponsive variants by two different selection schemes: one using a TGF alpha-toxin conjugate, and the other using a TGF alpha-inducible morphological phenotype. Each variant had decreased [125I]EGF specific binding and little or no EGF growth inhibitory response in vitro. We also isolated a mutagen-induced revertant from one of the variants based on expression of the TGF alpha-inducible morphological phenotype. These cells were found to have a normal complement of receptors and EGF growth inhibitory response in vitro. GH4C1, the two variants, and the revertant cells were inoculated into Wistar-Furth rats, and their growth observed for 8 weeks. The GH4C1 cells and the revertant formed tumors by 8 weeks, whereas the two variant cells failed to form tumors.(ABSTRACT TRUNCATED AT 400 WORDS)

Human pituitary somatotropes express transforming growth factor-alpha and its receptor

Transforming growth factor-alpha (TGF-alpha) is a growth-regulatory peptide produced by a variety of transformed and non-transformed cells. Among non-transformed cells, TGF-alpha has been identified in the prolactin (PRL)- and GH-secreting cells of the bovine anterior pituitary gland. In this report, we have examined the expression of TGF-alpha in human anterior pituitary glands by Western analysis and immunohistochemistry. For the Western analysis, human pituitary glands were extracted in acid/ethanol, an acetic acid-soluble fraction was ether-precipitated and dialysed, and TGF-alpha was partially purified by C18 chromatography. TGF-alpha was then identified by immunostaining of Western transfers. Anterior pituitary extracts exhibited a major band(s) migrating at 19 kDa that was immunoreactive with a monoclonal antibody directed against the mature TGF-alpha. However, no evidence of the fully processed 6 kDa TGF-alpha was observed. We next identified TGF-alpha by immunohistochemistry. Using both monoclonal and polyclonal antibodies, specific immunoreactivity was identified in a population of secretory cells in the anterior pituitary gland. Using antibodies specific for the COOH and NH3 terminals of the TGF-alpha precursor, a comparable number of TGF-alpha-positive cells were found to contain TGF-alpha precursor sequences. These results indicate that the 19 kDa form of TGF-alpha expressed in the human pituitary gland may exist as the transmembrane form. We next sought to determine which cells express TGF-alpha in a human male pituitary gland. On frontal sections, TGF-alpha-immunopositive cells were evenly distributed in a manner and number indistinguishable from GH-immunopositive cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of rapid and sensitive high throughput pharmacologic assays for marine phycotoxins

The lack of rapid, high throughput assays is a major obstacle to many aspects of research on marine phycotoxins. Here we describe the application of microplate scintillation technology to develop high throughput assays for several classes of marine phycotoxin based on their differential pharmacologic actions. High throughput "drug discovery" format microplate receptor binding assays developed for brevetoxins/ciguatoxins and for domoic acid are described. Analysis for brevetoxins/ciguatoxins is carried out by binding competition with [3H] PbTx-3 for site 5 on the voltage dependent sodium channel in rat brain synaptosomes. Analysis of domoic acid is based on binding competition with [3H] kainic acid for the kainate/quisqualate glutamate receptor using frog brain synaptosomes. In addition, a high throughput microplate 45Ca flux assay for determination of maitotoxins is described. These microplate assays can be completed within 3 hours, have sensitivities of less than 1 ng, and can analyze dozens of samples simultaneously. The assays have been demonstrated to be useful for assessing algal toxicity and for assay-guided purification of toxins, and are applicable to the detection of biotoxins in seafood.