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Biomedical subjects

S M Powell

Publications and source records attributed to S M Powell.

At least 19 recordsLinked to original sources
PubMed1992

APC mutations occur early during colorectal tumorigenesis.

Human tumorigenesis is associated with the accumulation of mutations both in oncogenes and in tumour suppressor genes. But in no common adult cancer have the mutations that are critical in the early stages of the tumorigenic process been defined. We have attempted to determine if mutations of the APC gene play such a role in human colorectal tumours, which evolve from small benign tumours (adenomas) to larger malignant tumours (carcinomas) over the course of several decades. Here we report that sequence analysis of 41 colorectal tumours revealed that the majority of colorectal carcinomas (60%) and adenomas (63%) contained a mutated APC gene. Furthermore, the APC gene met two criteria of importance for tumour initiation. First, mutations of this gene were found in the earliest tumours that could be analysed, including adenomas as small as 0.5 cm in diameter. Second, the frequency of such mutations remained constant as tumours progressed from benign to malignant stages. These data provide strong evidence that mutations of the APC gene play a major role in the early development of colorectal neoplasms.

Adenoma↗
PubMed1992

Cloning and characterization of the cDNA encoding human adenylosuccinate synthetase.

Adenylosuccinate synthetase (AS) catalyzes the first committed step in the conversion of IMP to AMP. A cDNA was isolated from a human liver library which encodes a protein of 455 amino acids (M(r) of 49,925). Alignments of human, mouse, Dictyostelium discoideum and E. coli AS sequences identify a number of invariant residues which are likely to be important for structure and/or catalysis. The human AS sequence was also 19% identical to the human urea cycle enzyme, argininosuccinate synthetase (ASS), which catalyzes a chemically similar reaction. Both human liver and HeLa AS mRNA showed signals of 2.3 and 2.8 kb. An unmodified N-terminus is required for function of the human AS enzyme in E. coli mutants lacking the bacterial enzyme. The human cDNA provides a means to assess the possible role of AS abnormalities in unclassified, idiopathic cases of gout.

Adenylosuccinate Synthase↗
PubMed1991

High incidence of contact dermatitis in leg-ulcer patients--implications for management.

A retrospective review of patch test results from all new patients with venous leg ulcers was performed for the preceding 11 months. Eighty one patients referred from general practitioners and district nurses with venous stasis ulcers were included. Positive patch tests were found in 54 patients (67%), including a continued high incidence of allergy to lanolin and topical antibiotics. Multiple allergies were found in 48 patients (58%). In addition, a new problem of allergy to cetearyl alcohol, a constituent of commonly used creams and paste bandages, was identified in 13 patients. There is a continuing high incidence of contact sensitivity in patients with venous stasis ulcers which has important implications for the management of these patients.

Anti-Bacterial Agents↗
PubMed1990

Sex-specific fetal lung development and müllerian inhibiting substance.

Male neonates develop respiratory distress syndrome (RDS) with a greater incidence and mortality than do female neonates; the cause of this male disadvantage remains obscure. Male fetuses are exposed to higher levels of androgens and Müllerian inhibiting substance (MIS). Androgens have been shown to inhibit fetal lung maturation, and recent evidence in vitro indicates that MIS, a Sertoli cell-derived glycoprotein made early in ontogeny of the testis, may also inhibit lung development. To study whether this fetal regressor might inhibit maturation of the fetal lung in vivo, we injected human recombinant MIS (rMIS) into fetal rats, measured serum levels of rMIS using an enzyme-linked immunosorbent assay, and analyzed fetal lung tissue histologically and for protein, glycogen, DNA, and disaturated phosphatidylcholine content. Peak serum levels of recombinant MIS were measured at 6 h, with an apparent elimination half-life of 3 h, and without leakage into adjacent littermates injected with vehicle alone. Female fetal rat lung tissue exposed to recombinant MIS (10(-9) M, 10(-8) M) revealed depressed disaturated phosphatidylcholine content both 48 and 72 h after injection compared with female vehicle-injected littermates. Male lungs of the same gestational age appeared inhibited at a higher (10(-8) M) rMIS dose. These inhibitory effects observed in vivo confirm those previously seen in vitro and suggest that MIS, as well as androgens, may play a causative or important ancillary role in the sexual dimorphism that characterizes the neonatal respiratory distress syndrome.

Animals↗
PubMed1989

Nutritional and hormonal regulation of the activity state of hepatic branched-chain alpha-keto acid dehydrogenase complex.

The hepatic branched-chain alpha-keto acid dehydrogenase complex plays an important role in regulating branched-chain amino acid levels. These compounds are essential for protein synthesis but are toxic if present in excess. When dietary protein is deficient, the hepatic enzyme is present in the inactive, phosphorylated state to allow conservation of branched-chain amino acids for protein synthesis. When dietary protein is excessive, the enzyme is in the active, dephosphorylated state to commit the excess branched-chain amino acids to degradation. Inhibition of protein synthesis by cycloheximide, even when the animal is starving for protein, results in activation of the hepatic branched-chain alpha-keto acid dehydrogenase complex to prevent accumulation of branched-chain amino acids. Likewise, the increase in branched-chain amino acids caused by body wasting during starvation and uncontrolled diabetes is blunted by activation of the hepatic branched-chain alpha-keto acid dehydrogenase complex. The activity state of the hepatic branched-chain alpha-keto acid dehydrogenase complex is regulated in the short term by the concentration of branched-chain alpha-keto acids (inhibitors of branched-chain alpha-keto acid dehydrogenase kinase) and in the long term by alteration in the total branched chain alpha-keto acid dehydrogenase kinase activity.

3-Methyl-2-Oxobutanoate Dehydrogenase (Lipoamide)↗
PubMed1986

Regulation of the branched-chain 2-oxo acid dehydrogenase complex in hepatocytes isolated from rats fed on a low-protein diet.

Hepatocytes isolated from rats fed on a chow diet or a low-protein (8%) diet were used to study the effects of various factors on flux through the branched-chain 2-oxo acid dehydrogenase complex. The activity of this complex was also determined in cell-free extracts of the hepatocytes. Hepatocytes isolated from chow-fed rats had greater flux rates (decarboxylation rates of 3-methyl-2-oxobutanoate and 4-methyl-2-oxopentanoate) than did hepatocytes isolated from rats fed on the low-protein diet. Oxidizable substrates tended to inhibit flux through the branched-chain 2-oxo acid dehydrogenase, but inhibition was greater with hepatocytes isolated from rats fed on the low-protein diet. 2-Chloro-4-methylpentanoate (inhibitor of branched-chain 2-oxo acid dehydrogenase kinase), dichloroacetate (inhibitor of both pyruvate dehydrogenase kinase and branched-chain 2-oxo acid dehydrogenase kinase) and dibutyryl cyclic AMP (inhibitor of glycolysis) were effective stimulators of branched-chain oxo acid decarboxylation with hepatocytes from rats fed on a low-protein diet, but had little effect with hepatocytes from rats fed on chow diet. Activity measurements indicated that the branched-chain 2-oxo acid dehydrogenase complex was mainly (96%) in the active (dephosphorylated) state in hepatocytes from chow-fed rats, but only partially (50%) in the active state in hepatocytes from rats fed on a low-protein diet. Oxidizable substrates markedly decreased the activity state of the enzyme in hepatocytes from rats fed on a low-protein diet, but had much less effect in hepatocytes from chow-fed rats. 2-Chloro-4-methylpentanoate and dichloroacetate increased the activity state of the enzyme in hepatocytes from rats fed on a low-protein diet, but had no effect on the activity state of the enzyme in hepatocytes from chow-fed rats. The results indicate that protein starvation greatly increases the sensitivity of the hepatic branched-chain 2-oxo acid dehydrogenase complex to regulation by covalent modification.

3-Methyl-2-Oxobutanoate Dehydrogenase (Lipoamide)↗
PubMed1986

Regulation of branched-chain alpha-ketoacid dehydrogenase complex by covalent modification.

The branched-chain alpha-ketoacid dehydrogenase complex, like the pyruvate dehydrogenase complex, is an intramitochondrial enzyme subject to regulation by covalent modification. Phosphorylation causes inactivation and dephosphorylation causes activation of both complexes. The branched-chain alpha-ketoacid dehydrogenase kinase, believed distinct from pyruvate dehydrogenase kinase, is an integral component of the branched-chain alpha-ketoacid dehydrogenase complex and is sensitive to inhibition by branched-chain alpha-ketoacids, alpha-chloroisocaproate, phenylpyruvate, clofibric acid, octanoate and dichloroacetate. Phosphorylation of branched-chain alpha-ketoacid dehydrogenase occurs at two closely-linked serine residues (sites 1 and 2) of the alpha-subunit of the decarboxylase. HPLC and sequence data suggest homology of the amino acid sequence adjacent to phosphorylation sites 1 and 2 of complexes isolated from several different tissues. Stoichiometry for phosphorylation of all of the complexes studies was about 1 mol P/mol alpha-subunit for 95% inactivation and 1.5 mol P/mol alpha-subunit for maximally phosphorylated complex. Site 1 and site 2 were phosphorylated at similar rates until total phosphorylation exceeded 1 mol P/mol alpha-subunit. The complexes from rabbit kidney, rabbit heart, and rat heart showed 30-40% additional phosphorylation of the alpha-subunit beyond 95% inactivation. Site specificity studies carried out with the kinase partially inhibited with alpha-chloroisocaproate suggest that phosphorylation of site 1 is primarily responsible for regulation of the complex. The capacity of the branched-chain alpha-ketoacid dehydrogenase to oxidize pyruvate (Km = 0.8 mM, Vmax = 20% of that of alpha-ketoisovalerate) interferes with the estimation of activity state of the hepatic pyruvate dehydrogenase complex. The disparity between the activity states of the two complexes in most physiologic states contributes to this interference. An inhibitory antibody for branched-chain alpha-ketoacid dehydrogenase can be used to prevent interference with the pyruvate dehydrogenase assay. Almost all of the hepatic branched-chain alpha-ketoacid dehydrogenase in chow-fed rats is active (greater than 90% dephosphorylated). In contrast, almost all of the hepatic enzyme of rats fed a low-protein (8%) diet is inactive (greater than 85% phosphorylated). Fasting of chow-fed rats has no effect on the activity state of hepatic branched-chain alpha-ketoacid dehydrogenase, i.e. greater than 90% of the enzyme remains in the active state. However, fasting of rats maintained on low-protein diets greatly activates the hepatic enzyme.(ABSTRACT TRUNCATED AT 400 WORDS)

3-Methyl-2-Oxobutanoate Dehydrogenase (Lipoamide)↗
PubMed1985

Physiological covalent regulation of rat liver branched-chain alpha-ketoacid dehydrogenase.

A radiochemical assay was developed for measuring branched-chain alpha-ketoacid dehydrogenase activity of Triton X-100 extracts of freeze-clamped rat liver. The proportion of active (dephosphorylated) enzyme was determined by measuring enzyme activities before and after activation of the complex with a broad-specificity phosphoprotein phosphatase. Hepatic branched-chain alpha-ketoacid dehydrogenase activity in normal male Wistar rats was 97% active but decreased to 33% active after 2 days on low-protein (8%) diet and to 13% active after 4 days on the same diet. Restricting protein intake of lean and obese female Zucker rats also caused inactivation of hepatic branched-chain alpha-ketoacid dehydrogenase complex. Essentially all of the enzyme was in the active state in rats maintained for 14 days on either 30 or 50% protein diets. This was also the case for rats maintained on a commercial chow diet (minimum 23% protein). However, maintaining rats on 20, 8, and 0% protein diets decreased the percentage of the active form of the enzyme to 58, 10, and 7% of the total, respectively. Fasting of chow-fed rats for 48 h had no effect on the activity state of hepatic branched-chain alpha-ketoacid dehydrogenase, i.e., 93% of the enzyme remained in the active state compared to 97% for chow-fed rats. However, hepatic enzyme of rats maintained on 8% protein diet was 10% active before starvation and 83% active after 2 days of starvation. Thus, dietary protein deficiency results in inactivation of hepatic branched-chain alpha-ketoacid dehydrogenase complex, presumably as a consequence of low hepatic levels of branched-chain alpha-ketoacids, established inhibitors of branched-chain alpha-ketoacid dehydrogenase kinase. With rats fed a low-protein diet and subsequently starved, inhibition of branched-chain alpha-ketoacid dehydrogenase kinase by branched-chain alpha-ketoacids generated as a consequence of endogenous proteolysis most likely promotes the greater branched-chain alpha-ketoacid dehydrogenase activity state.

3-Methyl-2-Oxobutanoate Dehydrogenase (Lipoamide)↗
PubMed1983

Mullerian inhibiting substance fractionation by dye affinity chromatography.

Mullerian inhibiting substance (MIS), a large glycoprotein secreted by the fetal and neonatal testis, is responsible for regression of the Mullerian ducts in the male embryo. This fetal growth regulator has been purified more than 2000-fold from crude testicular incubation medium following fractionation on a triazinyl dye affinity support. A high yield of 60% recovered activity was achieved in the absence of exogenous carrier protein by stabilizing MIS with 2-mercaptoethanol, EDTA, and Nonidet-P40 and eliminating losses in the handling and concentration of MIS fractions. Although affinity elution with nucleotides has proved successful in other systems, MIS could not be eluted with ATP, GTP, or AMP, with or without divalent metal ions. Nucleotide elution, however, does remove contaminating proteins prior to MIS recovery with high ionic strength. The 2000-fold-purified MIS fraction, although not homogeneous, shows a reduction-sensitive band after SDS-gel electrophoresis that has been proposed to be the MIS dimer.

Animals↗
PubMed1976

Glucose-6-phosphate dehydrogenase in lichen planus skin.

The apparent Michaelis constant (Km) for glucose-6-phosphate of the enzyme glucose-6-phosphate dehydrogenase has been measured in extracts prepared from biopsies of normal human skin and from both affected and apparently normal skin of patients with lichen planus. No differences of Km were found and starch gel electrophoresis of extracts from lichen planus lesions and normal controls showed similar patterns when stained for glucose-6-phosphate dehydrogenase activity. These results do not support the view that lichen planus is an inborn error of metabolism in which the structure of glucose-6-phosphate dehydrogenase of skin is affected.

Electrophoresis, Starch Gel↗
PubMed1976

The cutaneous reactions to kallikrein, prostaglandin and thurfyl nicotinate in chronic urticaria and the effect of polyphloretin phosphate.

The erythema and wealing resulting from the application of thurfyl nicotinate ointment (Trafuril) and from the inoculation of kallikrein has been studied in patients with chronic urticaria and normal controls. Polyphloretin phosphate (PPP) suppressed the reaction in controls but in patients with urticaria it increased the reactions to Trafuril and had little effect on the kallikrein reaction. PPP also suppressed the PGE2-induced erythma in normal controls but not in urticaria patients. In a separate study using fibrinolysis autography, prostaglandin (PG) E2 and PGF2alpha depressed fibrinolysis in the skin of two pigs and both kallikrein and Trafuril suppressed fibrinolysis in human skin. It is suggested that the inflammatory reaction induced by thurfyl nicotinate and kallikrein is mediated in part by a prostaglandin-like action. Several anomalies in the action of Trafuril in skin diseases can be explained if such prostaglandin-like activity is mediated in part through inhibition of fibrinolysis.

Chronic Disease↗