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M D Davis

Publications and source records attributed to M D Davis.

At least 109 records · Page 6Linked to original sources
PubMed1988

The auto-oxidation of tetrahydrobiopterin.

The product of the aerobic oxidation of tetrahydrobiopterin, quinonoid dihydrobiopterin, is unstable and rapidly rearranges to form a 7,8-dihydropteridine. Kaufman [Kaufman, S. (1967) J. Biol. Chem. 242, 3934-3943] identified the stable product produced in 0.1 M phosphate pH 6.8, as 7,8-dihydrobiopterin. However, Armarego et al. [Armarego, W. L. F., Randles, D. and Taguchi, H. (1983) Eur. J. Biochem. 135 393-403] questioned this assignment because they found that the dihydroxypropyl group on C-6 was eliminated and 7,8-dihydropterin was the predominant product when the aerobic oxidation was performed in 0.1 M Tris pH 7.6. In the present study we demonstrate that the rearrangement of the unstable quinonoid dihydrobiopterin results in a mixture of these two 7,8-dihydropteridines at neutral pH, 25 degrees C. Furthermore, we find that the loss or retention of the alkyl side-chain is not solely dependent on the pH of the reaction mixture, as was previously assumed by Armarego et al., but rather is strongly influenced by the temperature and the type of buffer. In addition, we describe a new method for quantifying the relative amounts of these two 7,8-dihydropteridines in mixtures of unknown concentrations. This method relies on multicomponent analysis of second derivative spectra and results in values which agree with the concentrations determined directly by HPLC.

Air↗
PubMed1988

Effect of alkaline pH on the activity of rat liver phenylalanine hydroxylase.

The pH optimum of rat liver phenylalanine hydroxylase is dependent on the structure of the cofactor employed and on the state of activation of the enzyme. The tetrahydrobiopterin-dependent activity of native phenylalanine hydroxylase has a pH optimum of about 8.5. In contrast, the 6,7-dimethyltetrahydropterin-dependent activity is highest at pH 7.0. Activation of phenylalanine hydroxylase either by preincubation with phenylalanine or by limited proteolysis results in a shift of the pH optimum of the tetrahydrobiopterin-dependent activity to pH 7.0. Activation of the enzyme has no effect on the optimal pH of the 6,7-dimethyltetrahydropterin-dependent activity. The different pH optimum of the tetrahydrobiopterin-dependent activity of native phenylalanine hydroxylase is due to a change in the properties of the enzyme when the pH is increased from pH 7 to 9.5. Phenylalanine hydroxylase at alkaline pH appears to be in an altered conformation that is very similar to that of the enzyme which has been activated by preincubation with phenylalanine as determined by changes in the intrinsic protein fluorescence spectrum of the enzyme. Furthermore, phenylalanine hydroxylase which has been preincubated at an alkaline pH in the absence of phenylalanine and subsequently assayed at pH 7.0 in the presence of phenylalanine shows an increase in tetrahydrobiopterin-dependent activity similar to that exhibited by the enzyme which has been activated by preincubation with phenylalanine at neutral pH. Activation of the enzyme also occurs when m-tyrosine or tryptophan replace phenylalanine in the assay mixture. The predominant cause of the increase in activity of the enzyme immediately following preincubation at alkaline pH appears to be the increase in the rate of activation by the amino acid substrate. However, in the absence of substrate activation, phenylalanine hydroxylase preincubated at alkaline pH displays an approximately 2-fold greater intrinsic activity than the native enzyme.

Animals↗
PubMed1987

Endogenous dopamine and serotonin release from the explanted rat tuberohypophyseal system: effects of electrical stimulation and neurotensin.

Excised blocks of brain tissue encompassing the hypothalamic periventricular nucleus, arcuate nucleus, infundibular stalk and attached pituitary neurointermediate lobe (NIL) were obtained from rats and perifused in vitro. The intact tuberohypophyseal dopaminergic pathway projecting from cell bodies in the hypothalamus to terminations in the pars intermedia and posterior lobes was thus isolated. An electrode placed in the explant delivered electrical pulses at various frequencies to hypothalamic targets. Products released in the immediate vicinity of the NIL were continuously collected and assayed for dopamine and serotonin content. Electrical stimulation of the arcuate nucleus from 1.0 - 10.0 Hz produced a frequency-dependent increase in dopamine, but not serotonin, release from the NIL. Stimulation of the infundibular stalk, however, elevated the release of both neurotransmitters. Addition of neurotensin (0.001 - 1.0 microM) to the bathing media produced a concentration-dependent increase in dopamine, but not serotonin, release. These experiments demonstrate the utility of the tuberohypophyseal explant as a model for use in the study of dopaminergic neuronal function in this neuroendocrine axis.

Animals↗
PubMed1987

The Wisconsin Epidemiologic Study of Diabetic Retinopathy. VII. Diabetic nonproliferative retinal lesions.

The prevalences of hard exudates, soft exudates, intraretinal microvascular abnormalities (IRMAs), and venous beading and their relationships to demographic and other characteristics were examined in a population-based study in southern Wisconsin. For participants whose age at diagnosis was less than 30 years and who were taking insulin (N = 996), hard exudates were found in 24.2%, soft exudates in 15.3%, IRMAs in 16.5%, and venous beading in 7.0% of the population. For participants whose age at diagnosis was 30 years or older and who were taking insulin (N = 674), hard exudates were found in 28.3%, soft exudates in 15.5%, IRMAs in 8.8%, and venous beading in 3.2%. For older-onset persons not taking insulin (N = 696), hard exudates were found in 9.4%, soft exudates in 5.4%, IRMAs in 2.6%, and venous beading in 0.9% of the population. The severities of the lesions were found to be consistently associated with longer duration of diabetes in younger-onset persons and the presence of proteinuria in older-onset persons.

Cross-Sectional Studies↗
PubMed1987

Macular thickening and visual acuity. Measurement in patients with cystoid macular edema.

Cystoid macular edema (CME) is commonly associated with many ocular conditions. The presence of CME on fluorescein angiographic examination need not, however, preclude good visual acuity. The hypothesis that the degree of macular thickening is associated with visual acuity was tested. Ten consecutive stereoscopic fluorescein angiograms were graded by 13 ophthalmologists using a set of four standards. Linear regression showed a significant relationship between mean macular thickening and the visual acuity recorded at the time of angiography. However, no significant relationship could be made between the estimation of visual acuity and the amount of fluorescein staining measured in the posterior pole. Although the observation of fluorescein leakage is indispensable for documenting a breakdown in the blood-retinal barrier, the observation of retinal thickening is important for identifying the sites of edema accumulation, and may be the useful parameter to follow when trying to assess improvement or worsening of retinal edema and in cases of uveitis when the cause of poor vision may be multifactorial.

Adult↗
PubMed1987

The Wisconsin Epidemiologic Study of Diabetic Retinopathy. VI. Retinal photocoagulation.

The prevalence of focal and panretinal photocoagulation and its relationship to demographic and other characteristics were examined in a population-based study in southern Wisconsin. For participants whose age at diagnosis was less than 30 years and who were taking insulin (996 persons) the prevalence rate of panretinal photocoagulation (13.9%) was higher than that of focal photocoagulation (3.6%). For those whose age at diagnosis was 30 years or older (1370 persons), the prevalence rate for panretinal photocoagulation (3.6%) was slightly higher than that of focal photocoagulation (3.0%). Seventy-two percent of eyes of younger onset and 45% of eyes of older onset persons that had received panretinal photocoagulation treatment were found to have incomplete regression of retinal new vessels, and in approximately half of these eyes severe proliferative retinopathy (Diabetic Retinopathy Study High Risk Characteristics [DRS-HRC]) was present. Among eyes with DRS-HRC, 55% were found to be untreated.

Adult↗
PubMed1987

Macular edema in Diabetic Retinopathy Study patients. Diabetic Retinopathy Study Report Number 12.

Results from the Diabetic Retinopathy Study (DRS) demonstrate that scatter photocoagulation is associated with some loss of visual acuity soon after treatment. This visual loss is especially prominent in eyes with preexisting macular edema. It is also associated with the intensity of treatment. Reducing macular edema by focal photocoagulation before initiating scatter treatment and dividing scatter treatment into multiple sessions with less intense burns may decrease the risk of the visual loss associated with photocoagulation.

Diabetic Retinopathy↗
PubMed1986

A modified ferrozine method for the measurement of enzyme-bound iron.

A general procedure for the determination of the iron content of enzymes by digestion with methanesulfonic acid to release protein-bound iron has been developed. This procedure replaces the tedious and potentially hazardous method of wet ashing with concentrated nitric-sulfuric-perchloric acids. The method has been used to determine the stoichiometry of iron for nanomole quantities of heme-iron proteins, iron-sulfur proteins, complex iron-sulfur proteins, as well as in phenylalanine hydroxylase, an enzyme with iron in an undetermined coordination.

Catalase↗
PubMed1986

An alternative method of grading diabetic retinopathy.

The purpose of this report is to present a system for grading the severity of diabetic retinopathy that is a rapid, relatively inexpensive, and standardized alternative to the more detailed Early Treatment Diabetic Retinopathy Study (ETDRS) system; present data on its reproducibility; and compare it to the detailed ETDRS grading system. The alternative system was used to grade fundus photographs obtained during a large prevalence study, the Wisconsin Epidemiologic Study of Diabetic Retinopathy (WESDR). The alternative method involved grading seven stereoscopic standard fields as a whole, and assigning a level of severity for the eye according to the greatest degree of retinopathy using a modified Airlie House Classification scheme. Using an eight-level classification system of increasing severity of retinopathy, there was 78.3% exact agreement between the alternative and ETDRS systems. A grader regraded all 503 disagreements, and was in exact agreement 49.3% of the time with the alternative system, 35.7% of the time with the detailed system, and 15.0% with neither the alternative or detailed systems. Interobserver agreement for the alternative system was 78.5%; intraobserver agreement over a 9 month to 1 year period was 90.0% for grader 1 and 84.0% for grader 2. The alternative system of grading, when used by experienced graders, is a reproducible method for objectively determining retinopathy status in epidemiologic studies.

Diabetic Retinopathy↗
PubMed1986

The hypothalamo-hypophyseal rat explant in vitro: endocrinological studies of the pars intermedia dopaminergic neural input.

Short-term in vitro incubation of hypothalamo-hypophyseal tissue from young rats was undertaken to discern more clearly the functional relationship between putative dopaminergic neural projections in the pars intermedia and the secretory activity of melanophore stimulating hormone (MSH). This explant consisted of a portion of the mediobasal hypothalamus containing the dopamine neurone cell bodies of interest, with the attached pituitary neuro-intermediate lobe (n.i.l.). The n.i.l. was inserted into the end of a 1 mm diameter tube attached to a perfusion pump which allowed uninterrupted sampling of medium neighbouring the n.i.l. A 'real-time' analysis of hormone secretion was obtained by immediately and continuously bioassaying for MSH. A bipolar stimulating electrode was placed on the ventral floor of the mediobasal hypothalamus either directly on the arcuate nucleus, median eminence or infundibular stalk. Electrical stimulation for 5 min (0.1-20.0 Hz) caused a transient inhibition of basal MSH secretion, while continuous stimulation (0.1-5.0 Hz) led to a much greater, long-term, reversible inhibition. In the latter, the degree of inhibition was generally dependent on stimulation rate up to a maximum at 5 Hz. Application of the dopamine D2 receptor antagonist, 1-sulpiride (0.001-0.1 microM) to the perfusion medium not only completely and reversibly blocked the stimulus-induced inhibition of MSH release but by itself, significantly increased the basal secretion rate. Applied to the isolated n.i.l., 1-sulpiride did not alter release but did prevent the inhibitory response caused by exogenously applied dopamine (0.1 microM). The gamma-aminobutyric acid receptor antagonist, bicuculline (0.01-1.0 microM), had no effect on any of the parameters studied. In explants, cutting the infundibular stalk linking the mediobasal hypothalamus with the n.i.l., mimicked the effects of 1-sulpiride by interrupting impulse flow to the gland. Thus, electrical stimulation of hypothalamic neurones in these explants apparently causes a release of dopamine from nerve terminals in the pars intermedia to inhibit MSH secretion and perhaps other pro-opiomelanocortin-derived peptides as well.

Animals↗
PubMed1985

The hypothalamohypophyseal system in vitro: electrophysiology of the pars intermedia and evidence for both excitatory and inhibitory inputs.

The purpose of this study was to better assess the function of catecholamine-containing nerve terminals in the pituitary pars intermedia lobe. Hypothalamohypophyseal explants, which included the intact mediobasal hypothalamus (MBH), median eminence, infundibular stalk and the neurointermediate lobe, were obtained from 2-3-week-old male and female albino rats. The tissue was placed in a perfusion chamber and maintained under physiological conditions for up to 12 h. A set of bipolar stimulating electrodes was positioned on the surface of the median eminence, infundibular stalk or the rostroventral arcuate nucleus of the MBH. A microelectrode recorded electrical activity in the pars intermedia gland. Two types of spontaneous action potentials were found; fast 2-4 ms duration neural fiber type spikes and slower 7-10 ms duration spikes probably derived from non-neural endocrine cells. Single-pulse electrical stimulation at all 3 sites evoked both kinds of potentials, while trains of stimuli (0.1-20 Hz) decreased or completely inhibited the basal firing rate of the slower ones. Application of the neuroleptic. L-sulpiride (0.01, 0.1 or 1.0 mumol), to the perfusion medium increased the spontaneous endocrine cell activity and blocked the stimulus-induced inhibition in the explants but had no effect on the activity in isolated pituitaries. Dopamine (0.1 mumol), which is known to inhibit the secretion of pro-opiomelanocortin peptides, reversibly suppressed the spontaneous endocrine cell potentials. These observations support a hypothesis for the presence of a functional tuberohypophyseal dopamine inhibitory system and a possible, but as yet unidentifiable, excitatory system in the pars intermedia. Thus, hypothalamohypophyseal explants can be used to elucidate specific information on this type of neuroendocrine axis.

Action Potentials↗
PubMed1985

The Wisconsin Epidemiologic Study of Diabetic Retinopathy. A comparison of retinopathy in younger and older onset diabetic persons.

In a population-based survey of diabetic persons, retinopathy was detected by stereoscopic color fundus photography in 70% of persons under 30 years of age at diagnosis and taking insulin (Group YO), in 62% of persons 30 years of age or older at diagnosis and taking insulin (Group OO-I) and in 36% of persons 30 years of age or older at diagnosis not taking insulin (Group OO-N). The mean duration of known diabetes was 14.6 years in Group YO, 11.0 years in Group OO-I and 6.9 years in Group OO-N. After 20 years of diabetes, proliferative retinopathy was present in about 50% of Group YO, about 25% of Group OO-I and about 5% of Group OO-N. After 15 years of diabetes, macular edema was present in about 18% of Group YO, about 20% of Group OO-I and about 12% of Group OO-N. When present, macular edema tended to be associated with more hard exudate in Group OO-N.

Adult↗
PubMed1985

Conference on insulin pump therapy in diabetes. Multicenter study effect on microvascular disease. Studies of retinopathy. Methodology for assessment and classification with fundus photographs.

A major goal of the study was to assess the methodologies available for the quantification of retinal lesions suitable for application in trials of metabolic management. We describe the methods used for grading stereoscopic color fundus photographs. Overall retinopathy severity level for each eye ranging from normal (level 10) to high-risk category (level 70) was assigned on the basis of gradings of individual lesions. Change in retinopathy severity level over the 8-mo period was detected in 3-62% of patients, depending on the method used and the definition of change. Lesions responsible for change in level were mainly retinal infarcts (soft exudates) and intraretinal microvascular abnormalities.

Diabetic Retinopathy↗
PubMed1984

31P nuclear magnetic resonance and chemical studies of the phosphorus residues in bovine milk xanthine oxidase.

In addition to the phosphate residues contained in the acid-dissociable FAD and the molybdenum cofactor moieties, milk xanthine oxidase contains one mole of covalently bound phosphorus per active-center molybdenum. Acid hydrolysis of the apoprotein moiety and subsequent analysis by high-voltage thin-layer electrophoresis has identified the phosphorylated amino acid residue to be phosphoserine. 31P NMR data show the phosphopeptide to be monosubstituted, in agreement with the chemical analysis. A pH-dependent chemical shift of the phosphorus residue in the molybdenum cofactor moiety is also observed which provides unequivocal support for suggestions in the literature that this cofactor contains a monosubstituted phosphate. 31P NMR studies on the intact enzyme show phosphorus resonances at about -3 ppm, +1 ppm, +8.8 ppm and at +13.5 ppm. The resonances at +8.8 ppm and at +13.5 ppm are assigned to those of the pyrophosphate linkage of the FAD moiety by analogy with chemical shift data of the FAD on glucose oxidase [James, T.L., Edmondson, D.E., and Husain, M. (1981) Biochemistry 20, 617] and from the absence of any resonances in this region upon examination of preparations of deflavo xanthine oxidase. The intensity and resolution of the resonance at about -3 ppm is dependent on the degree of functionality of the enzyme. This resonance has a small amplitude relative to the FAD resonances in 50-60% functional enzyme, but increases dramatically in intensity in the desulpho enzyme. This resonance is the only one exposed to solvent as it is the only one susceptible to paramagnetic line-broadening on the addition of Mn(II) to the enzyme solution. Treatment of the enzyme with allopurinol leads to alteration of the approximately equal to -3-ppm resonance, but does not significantly affect the other resonances. Formation of the stable Mo(V) 'inhibited' form of the enzyme with ethylene glycol results in extensive line-broadening of the resonances at -3 ppm and +1 ppm, but has no observable affect on the FAD resonances. These data suggest that in addition to the phosphate on the molybdenum cofactor, the phosphoserine residue in xanthine oxidase is also in close proximity to the activesite molybdenum center of this enzyme. These results are discussed with respect to possible implications on the catalytic mechanism of the enzyme.

Animals↗
PubMed1984

The reaction of xanthine oxidase with lumazine. Characterization of the reductive half-reaction.

The optical and fluorescence characteristics of lumazine (2, 4-dihydroxypteridine) and violapterin (2,4,6-trihydroxypteridine) are described. The reduction of xanthine oxidase by excess lumazine is markedly biphasic. The initial rapid phase is about 100-fold more rapid than Vmax and reflects the reaction of the enzyme with the first equivalent of lumazine; it is accompanied by the appearance of long wavelength absorbance of the Mo(IV) violapterin charge transfer species together with partial reduction in absorbance at 450 nm, reflecting a partial transfer of electrons from the Mo to the flavin and iron-sulfur center. A loss of substrate fluorescence and a burst of fluorescence due to enzyme bound product also occur during this phase. The second phase is rate-controlled by the dissociation of violapterin from the reduced enzyme. It is accompanied by a variable loss of the charge-transfer band, further reduction of the flavin and iron-sulfur chromophores and of substrate fluorescence, and the appearance of fluorescence of unbound product. This phase correlates well with the steady state parameters. The variation in kinetic behavior with pH is qualitatively explained by a marked decrease in the affinity of the enzyme as the pH is raised; Vmax is almost unaffected by pH. The temporal sequence of kinetic events reveals at least three steps in the reaction of xanthine oxidase with the first equivalent of lumazine.

Anaerobiosis↗
PubMed1984

The Wisconsin epidemiologic study of diabetic retinopathy. II. Prevalence and risk of diabetic retinopathy when age at diagnosis is less than 30 years.

In a population-based study in southern Wisconsin, 996 insulin-taking, younger-onset diabetic persons were examined using standard protocols to determine the prevalence and severity of diabetic retinopathy and associated risk variables. The prevalence of diabetic retinopathy varied from 17% to 97.5% in persons with diabetes for less than five years and 15 or more years, respectively. Proliferative retinopathy varied from 1.2% to 67% in persons with diabetes for less than ten years and 35 or more years, respectively. For persons with diabetes of 10 years' duration or less, the Cox regression model relates the severity or retinopathy to longer duration, older age at examination, and higher levels of glycosylated hemoglobin. After ten years of diabetes, severity of retinopathy was related to longer duration, high levels of glycosylated hemoglobin, presence of proteinuria, higher diastolic BP, and male sex.

Adolescent↗
PubMed1984

The Wisconsin epidemiologic study of diabetic retinopathy. III. Prevalence and risk of diabetic retinopathy when age at diagnosis is 30 or more years.

In a population-based study in southern Wisconsin, 1,370 patients given diagnoses of diabetes at age 30 years or older were examined using standard protocols to determine the prevalence and severity of diabetic retinopathy and associated risk variables. The prevalence of diabetic retinopathy varied from 28.8% in persons who had diabetes for less than five years to 77.8% in persons who had diabetes for 15 or more years. The rate of proliferative diabetic retinopathy varied from 2.0% in persons who had diabetes for less than five years to 15.5% in persons who had diabetes for 15 or more years. By using the Cox regression model, the severity of retinopathy was found to be related to longer duration of diabetes, younger age at diagnosis, higher glycosylated hemoglobin levels, higher systolic BP, use of insulin, presence of proteinuria, and small body mass.

Aged↗