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Biomedical subjects

K C Leung

Publications and source records attributed to K C Leung.

At least 37 records · Page 2Linked to original sources
PubMed1997

Human growth hormone fragment (hGH44-91) produces insulin resistance and hyperinsulinemia but is less potent than 22 kDa hGH in the rat.

A 17 kDa fragment of human growth hormone (22 kDa hGH), identified as hGH44-191, has lower binding affinity for growth hormone receptors (GHRs), but has been reported to be more potent in producing glucose intolerance in yellow obese mice. Out aim was to investigate this anomaly by comparing acute development of hyperinsulinemia and insulin resistance ("diabetogenic activity") during hGH44-191 or 22 kDa hGH infusion in normal rats. Fasted awake make rats (350-370 g) were infused via a carotid cannula with saline (CON), 22 kDa hGH (at 0.125 micrograms/min), or hGH44-191 (at 0.64 or 0.32 micrograms/min) for 5.75 h. Over the last 2 h, a euglycemic hyperinsulinemic clamp (insulin infusion rate 0.25 U/kg/h) was performed. After 3.75 h infusion, 22 kDa hGH at 0.125 and hGH44-191 at 0.64 micrograms/min produced basal (preclamp) hyperinsulinemia compared to CON. During the clamp, insulin resistance was consistently produced by 22 kDa hGH at 0.125 and hGH44-191, at 0.64 micrograms/min compared to CON. Using specific radioimmunoassays for 22 kDa hGH and hGH44-191, we determined that under conditions of equivalent diabetogenic activity, molar circulating levels of hGH44-191 were 50-60-fold higher than 22 kDa hGH. It was concluded that whereas 22 kDA hGH and hGH44-191 are both capable of generating acute hyperinsulinemia and insulin resistance in the normal rat, the diabetogenic potency of hGH44-191 is not enhanced compared to 22 kDa hGH, and that diabetogenic potency is in accord with the reported lower binding affinity of hGH44-191 to the GHR.

Animals↗
PubMed1997

Stimulation of mitochondrial fatty acid oxidation by growth hormone in human fibroblasts.

In vivo administration of GH induces lipolysis and lipid oxidation. However, it is not clear whether the stimulation of lipid oxidation is a direct effect of GH or is driven by increased substrate supply secondary to lipolysis. An in vitro bioassay has been established for assessing beta-oxidation of fatty acids in mitochondria, based on the measurement of conversion of tritiated palmitic acid to 5H2O by fibroblasts in culture. We have modified this assay to investigate whether GH stimulates fatty acid oxidation. GH stimulated oxidation of palmitic acid maximally by 26.7 +/- 2.5% (mean +/- SEM; P < 0.0001). The stimulation was biphasic, with the oxidation rate increasing with increasing GH concentration to a peak response at 1.5 nmol/L and declining to a level not significantly different from control thereafter. Insulin-like growth factor-I at concentrations of up to 250 nmol/L had no significant effect on fatty acid oxidation. GH-binding protein attenuated the effect of GH. An anti-GH receptor (GHR) antibody (MAb263), which dimerizes the receptor and induces GH-like biological actions, significantly stimulated fatty acid oxidation. Another anti-GHR antibody (MAb5), which prevents receptor dimerization, suppressed GH action. In summary, GH directly stimulated fatty acid oxidation, an action not mediated by insulin-like growth factor-I. Dimerization of GHRs was necessary for this effect. This bioassay is a practical tool for studying the regulatory effects of GH on lipid oxidation.

Antibodies, Monoclonal↗
PubMed1996

A highly sensitive growth hormone (GH) enzyme-linked immunosorbent assay uncovers increased contribution of a tonic mode of GH secretion in adults with organic GH deficiency.

The limited sensitivity of conventional GH assays has impeded a better understanding of the pathophysiology of GH secretion. In normal subjects, interpulse GH levels often fall below assay sensitivity, making it unclear whether secretion stops or is maintained at a tonic level below assay detectability. In patients with severe organic GH deficiency (GHD), GH levels are mostly undetectable. Using an ultrasensitive GH enzyme-linked immunosorbent assay to measure 24-h integrated GH concentrations, we recently provided evidence that these patients secrete low, but measurable, amounts of GH. In this report, we apply the same assay to characterize and compare 24-h GH profiles obtained by 20-min sampling in 10 subjects with organic GHD and 10 normal subjects matched for age, sex, and body mass index. With deconvolution analysis, which provides estimates of GH secretion and half-life, our aim was to determine 1) whether normal GH secretion is exclusively pulsatile, 2) how GH is secreted in subjects with organic GHD, and 3) the attributes of GH secretion that determine circulating insulin-like growth factor I (IGF-I) levels. All samples, including nadirs, from GHD subjects were well within the assay detection limit (1 ng/L). Peak 24-h GH levels in GHD subjects were lower and did not overlap those in the normal subjects. Nadir GH concentrations were significantly lower in GHD subjects (14 +/- 5 vs. 43 +/- 9 ng/L; P = 0.008), but the range overlapped that of normal subjects. Endogenous GH half-life did not differ significantly between the two groups. Normal subjects secreted GH in a mixed pulsatile and tonic mode, with pulsatile secretion accounting for 93 +/- 2% of the total production. Total daily GH production in GHD was approximately 5% of the production in matched normal subjects. This difference resulted from a greater reduction in the pulsatile (by 96%) than in the tonic (by 47%) component, so that the fractional daily contribution by tonic GH release in GHD subjects was markedly greater. There was a significant relationship between pulsatile GH secretion and serum IGF-I levels for the two groups combined. In summary, 1) peak, but not nadir, GH levels were completely segregated between GHD and normal subjects; and 2) although normal subjects secrete GH in a tonic and pulsatile mode, both modes are reduced in organic GHD, with a proportionately greater reduction in pulsatile secretion. We conclude that 1) nadir GH levels are not sufficiently discriminatory to be useful for the diagnosis of GH deficiency; 2) normal GH profiles arise from a mixed pattern of tonic and pulsatile secretion, whereas reduced GH secretion in organic GHD arises primarily from a marked diminution in the amount of pulsatile GH release; and 3) pulsatile GH release is a significant regulator of the IGF-I level in normal and GHD subjects.

Adult↗
PubMed1996

Effective doses for coronary angiography.

A detailed study of dose data has been made for 100 coronary angiography examinations performed on a digital X-ray unit. Dose-area product (DAP) data have been analysed in terms of fluoroscopy and radiography for different projections. Projections patterns were similar for all cardiologists studied and the contribution to DAP from fluoroscopy was 32%. Data for 10 patients, selected because the percentage contributions to DAP from fluoroscopy and radiography were similar to the mean for the group, were used to derive conversion factors from DAP to effective dose. The mean DAP was 14 Gy cm2 and the effective dose 3.1 mSv. Conversion factors, based on the same protocol, were derived for a range of tube potentials and filtrations for use in estimation of effective doses with other units.

Coronary Angiography↗
PubMed1995

Measurement of growth hormone-binding protein in the rat by a ligand immunofunctional assay.

We have developed a ligand immunofunctional assay (LIFA) for quantifying the circulating functional GH-binding protein (GHBP) in the rat. This two-site solid-phase assay uses a capture monoclonal antibody (4.3) specific to the hydrophilic C-terminal segment of rat GHBP (rGHBP), saturation of binding with human GH, and a detection system of rabbit antihuman GH polyclonal antibody and peroxidase-conjugated antirabbit immunoglobulin G antibody. Results were compared with Scatchard estimates derived by immuno-precipitation with monoclonal antibody 4.3. This assay was used to determine the GHBP levels in male and female rats and to investigate the diurnal properties and dynamics of GH and GHBP interaction in 15-min blood sampling over a 6-h period. The dynamic range of the rLIFA was 0.15-20.0 nM recombinant rGHBP, with intraassay and interassay coefficients of variation of 10.5% (n = 20) and 12.9% (n = 12), respectively. Serum GHBP levels determined by the rLIFA and those derived from Scatchard estimates were strongly correlated (n = 8; beta = 0.55; r2 = 0.89; P = 0.0005). Male rats had lower GHBP levels (6.5 +/- 0.7 nM; mean +/- SE; n = 14) than female rats (35.4 +/- 2.7 nM; n = 15; P = 0.0001). In the diurnal study, male rats had higher GH peaks (312.5 +/- 121.6 ng/ml; n = 7) than female rats (96.5 +/- 15.4 ng/ml; n = 9; P < 0.0001). In contrast to the pulsatile secretion of GH, GHBP levels in both sexes remained stable and showed no relationship to secretory pulses of GH. However, the GH bursts significantly altered the distribution of the GH-GHBP complex in male rats. By saturation and mass analysis, the greater GH pulsatile secretion in male rats resulted in occupancy of GHBP from less than 5% at nadir to about 80% at secretory peaks, in contrast to the less than 5-15% range of GHBP occupancy in female rats. In male rats, greater than 80% of GH at secretory peaks existed in the free form, whereas in female rats, 16-23% of GH existed in the free form during pulsatile secretion. In summary, the rLIFA shows good correlation to Scatchard analysis using an identical antibody. We conclude that this assay provides a rapid, sensitive, and accurate measurement of the circulating functional GHBP in the rat, and that it facilitates the study of GH and GHBP dynamics under a range of physiological conditions.

Animals↗
PubMed1995

Evaluation and application of a highly sensitive assay for serum growth hormone (GH) in the study of adult GH deficiency.

Previous work from our laboratory addressing the diagnosis of GH deficiency in adults showed that RIA measurement of the 24-h integrated GH concentration (IGHC) was unable to discriminate between hypopituitary and age-, sex-, and body mass index-matched normal subjects because of the occurrence of undetectable levels (< 200 ng/L) within both groups. In contrast, full separation was achieved using stimulation by the insulin tolerance test (ITT). The data showed no significant relationship between IGHC and insulin-like growth factor-I (IGF-I) within either group. To determine whether limited sensitivity obscured diagnostic and physiological information, we assessed and modified a commercially available enzyme-linked immunosorbent assay (Elegance GH ELISA, Bioclone Australia) to achieve a high sensitivity (1 ng/L) and applied it to the study of IGHC and the relationship to IGF-I in a study group of 30 normal and 19 subjects with severe organic hypopituitarism. Using this assay, the IGHCs from all subjects were detectable and correlated significantly with detectable values obtained by RIA (n = 24; r = 0.80; P = 0.0001). Mean IGHC in normal subjects was significantly higher than that in hypopituitary subjects (852 +/- 131 vs. 97 +/- 28 ng/L), but the IGHCs from the two groups were not completely separate. Twenty-six percent of hypopituitary subjects had IGHC values within the normal range (111-3454 ng/L). IGHC decreased with age in normal subjects. Age stratification improved the separation, but an overlap remained in the young (< 50 yr old) and old (> 50 yr old) groups. Measurement of 12-h nocturnal IGHC levels improved the separation between hypopituitary and normal subjects in the young subjects only. IGHC was significantly related to IGF-I in hypopituitary (r = 0.59; P = 0.0084) and normal subjects (r = 0.55; P = 0.0017) and in the combined groups (r = 0.64; P = 0.0001). The data show that a sensitive ELISA reliably quantifies IGHC in normal and hypopituitary subjects. IGHCs in hypopituitary patients are lower, but not clearly separated from values in normal counterparts despite their having unequivocally impaired GH responses to ITT. We conclude that 1) IGHC in normal subjects can be reliably defined by sensitive ELISAs; 2) the diagnostic utility of the IGHC does not match the reliability or simplicity of an ITT, and 3) GH is a significant regulator of IGF-I in both normal and reduced states of GH secretion.

Adolescent↗
PubMed1995

Do androgens regulate growth hormone-binding protein in adult man?

To determine whether adult serum GH-binding protein (GHBP) is regulated by androgen, serum GHBP concentrations were compared between 20 normal and 18 hypogonadal men matched for age and body mass index, and the effect of im testosterone treatment (250 mg testosterone enanthate) on GHBP levels in the 18 hypogonadal men was studied. Nine of the hypogonadal subjects had coexistent GH deficiency. Serum GHBP concentration was measured by a ligand immunofunctional assay. The mean serum GHBP level in untreated hypogonadal men was not significantly different from that of normal men (0.98 +/- 0.15 vs. 1.17 +/- 0.16 nmol/L). The mean serum insulin-like growth factor I (IGF-I) level was significantly lower in the hypogonadal men (132 +/- 22 vs. 206 +/- 17 ng/mL; P < 0.01). Basal testosterone (3.7 +/- 0.7 nmol/L) in hypogonadal men increased during treatment to a mean level of 29.1 +/- 2.8 nmol/L, which was not significantly higher than that in normal men (22.6 +/- 1.9 nmol/L). The mean serum GHBP level in hypogonadal men fell significantly during treatment to 0.60 +/- 0.11 nmol/L (P = 0.0003), whereas the serum IGF-I level rose significantly to 151 +/- 26 ng/mL (P < 0.04). The decrease in GHBP level was significant in both the GH-sufficient and GH-deficient subjects (P < 0.02 in both instances), whereas the increase in IGF-I level was significant in the GH-sufficient group (199 +/- 22 to 235 +/- 29 ng/mL; P < 0.04) but not in the GH-deficient group (53 +/- 7 to 55 +/- 5 ng/mL; P > 0.8). Thus, serum GHBP is normal in hypogonadal men but is reduced by testosterone treatment irrespective of endogenous GH-secretory status. It was concluded that the effect of testosterone on GHBP is pharmacological and occurs independent of GH mediation.

Adolescent↗
PubMed1993

Pregnancy and fetal long-chain 3-hydroxyacyl coenzyme A dehydrogenase deficiency.

We report on eleven pregnancies in 5 mothers. 6 of the babies had long-chain 3-hydroxyacyl coenzyme A dehydrogenase (LCHAD) deficiency, and each of the pregnancies was complicated by features such as fatty liver and HELLP (haemolysis, elevated liver enzymes, low platelets) syndrome. By contrast, 3 of the mothers also gave birth to unaffected babies, and these pregnancies were largely uncomplicated. We conclude that there may be adverse effects on maternal liver function from a fetus with LCHAD deficiency. Heterozygosity in the mother cannot alone account for the adverse effects because of the segregation of these effects with fetal LCHAD status.

3-Hydroxyacyl CoA Dehydrogenases↗
PubMed1992

A fatal neonatal case of medium-chain acyl-coenzyme A dehydrogenase deficiency with homozygous A-->G985 transition.

A term neonate became lethargic and hypotonic at 46 hours of age and died 10 hours later despite supportive therapy. Urinary organic acids indicated medium-chain acyl-coenzyme A dehydrogenase deficiency, and DNA studies confirmed this disorder. Neonatal symptoms in this enzyme deficiency have rarely been reported, and recent reviews have ignored or discounted this presentation.

Acyl-CoA Dehydrogenase↗
PubMed1991

Measurement of urokinase-type plasminogen activator activity in sera of nasopharyngeal carcinoma patients by an immunocapture assay.

The urokinase-type plasminogen activator (uPA) activity in sera of patients suffering from nasopharyngeal carcinoma was examined by a uPA-specific immunocapture assay. The results revealed that the activity levels in the patient sera were significantly higher than that of normal healthy controls. The uPA activity also increased with the staging of the disease and the anti-EBV VCA IgA titre which is a diagnostic test for the disease, although the results were not statistically different. These findings showed that the levels of the enzyme were related to the spread of the disease. In addition, it was demonstrated that the sensitivity of the assay could be increased by using SDS. If the sera were treated with 0.2% sodium dodecyl sulphate (SDS) prior to the assay, higher levels of measureable PA activity were detected.

Antigens, Viral↗
PubMed1989

Activation of pro-urokinase and plasminogen on human sarcoma cells: a proteolytic system with surface-bound reactants.

Human HT-1080 fibrosarcoma cells produce urokinase-type plasminogen activator (u-PA) and type 1 plasminogen activator inhibitor (PAI-1). We found that after incubation of monolayer cultures with purified native human plasminogen in serum-containing medium, bound plasmin activity could be eluted from the cells with tranexamic acid, an analogue of lysine. The bound plasmin was the result of plasminogen activation on the cell surface; plasmin activity was not taken up onto cells after deliberate addition of plasmin to the serum-containing medium. The cell surface plasmin formation was inhibited by an anticatalytic monoclonal antibody to u-PA, indicating that this enzyme was responsible for the activation. Preincubation of the cells with diisopropyl fluorophosphate-inhibited u-PA led to a decrease in surface-bound plasmin, indicating that a large part, if not all, of the cell surface plasminogen activation was catalyzed by surface-bound u-PA. In the absence of plasminogen, most of the cell surface u-PA was present in its single-chain proenzyme form, while addition of plasminogen led to formation of cell-bound two-chain u-PA. The latter reaction was catalyzed by cell-bound plasmin. Cell-bound u-PA was accessible to inhibition by endogenous PAI-1 and by added PAI-2, while the cell-bound plasmin was inaccessible to serum inhibitors, but accessible to added aprotinin and an anticatalytic monoclonal antibody. A model for cell surface plasminogen activation is proposed in which plasminogen binding to cells from serum medium is followed by plasminogen activation by trace amounts of bound active u-PA, to form bound plasmin, which in turn serves to produce more active u-PA from bound pro-u-PA. This exponential process is subject to regulation by endogenous PAI-1 and limited to the pericellular space.

Cell Line↗
PubMed1989

SED88: a Pascal program for the analysis of sedimentation equilibrium data.

Analytical ultracentrifugation is commonly used for the determination of molecular weights (sedimentation equilibrium) and sedimentation coefficients (sedimentation rate) of biological macromolecules in solution. A Turbo Pascal program for the analysis of sedimentation equilibrium centrifugation data produced by absorbance optical systems is described. The user may enter data from a scan of absorbance versus distance from the centre of rotation, via a graphics tablet (or ASCII file). This is subsequently manipulated to yield an apparent weight average molecular weight for the given sample. Plots of ln (absorbance) versus (radius2) may also be produced. The method described uses readily available computational equipment requiring only a graphics tablet in addition to an IBM PC compatible computer. This technique and the software developed have been used to investigate the molecular weight range of two International Humic Substances Society (IHSS) reference samples from the Suwannee River.

Absorption↗
PubMed1989

Decontamination of landfill leachate by soil infiltration.

A soil column study was conducted to evaluate the efficiency of soil to attenuate pollutants in landfill leachate. It was found that more than 60% of the initial alkalinity, COD, and total nitrogen was removed after the leachate had percolated through the soil column. Lower efficiencies were observed for the removal of dissolved cations, ammonia nitrogen, and nitrate nitrogen. When previously percolated leachate was recirculated through the soil column, significant removal (93.6%) of ammonia nitrogen was achieved. Moreover, a further reduction in COD, total nitrogen, nitrate nitrogen, and potassium was evident. Bioassay tests of seed germination and root growth of Brassica chinensis were performed to compare the phytotoxicities of untreated, percolated, and recirculated leachates. The phytotoxicity was reduced by both treatments, with the greatest detoxification observed in the leachate recirculation treatment. The values of pH and Olsen phosphorus were significantly reduced (P less than 0.01) in the treated column soil, whereas significant increases (P less than 0.05) in electrical conductivity (EC), various forms of nitrogen, exchangeable sodium percentage (ESP), potassium, magnesium, zinc, and iron were observed. EC, ESP, potassium, magnesium, sodium, iron, and various forms of nitrogen accumulated at the soil surface (10 cm). On the other hand, manganese was deposited at 20 to 50 cm below the soil surface of the treated column.

Biological Assay↗
PubMed1987

Microplate immunocapture assay for plasminogen activators and their specific inhibitors.

We report a convenient sensitive enzyme activity assay for urokinase and tissue-type plasminogen activators, based on a solid-phase microtitre plate method using readily available polyclonal antibodies. The sensitivities for urokinase (active and proenzyme) and tissue activator were better than 1 ng/ml. The specificity was very high, with no significant contribution of urokinase in tissue activator assays or vice versa. This method is particularly useful for the assay of urokinase proenzyme in samples containing inhibitors. We describe how this assay may also be used to measure specific inhibitors of plasminogen activators, making use of their rapid formation of stable complexes with solid-phase activator. Inhibitors may be assayed in samples containing proenzymes.

Binding, Competitive↗
PubMed1987

The resistance of fibrin-stimulated tissue plasminogen activator to inactivation by a class PAI-2 inhibitor (minactivin).

Solid phase fibrin was an efficient stimulator of the tissue-type plasminogen activator (t-PA), and the plasmin produced could be detected by colorimetric assay of the soluble phase above the fibrin. However the fibrin-stimulated activity of t-PA was not inhibited by minactivin. This result was in contrast to that obtained with poly-D-lysine (PL) stimulated t-PA, where minactivin was a potent inhibitor. However, if PL was added to fibrin-bound t-PA, the enzyme once again became susceptible to minactivin inhibition. This occurred without release of t-PA from the fibrin matrix. Minactivin alone did not bind to fibrin or to the t-PA fibrin complex. It was therefore concluded that minactivin normally has no significant role in the regulation of t-PA mediated fibrinolysis, but this effect can be induced by PL.

Drug Interactions↗
PubMed1987

Poly-D-lysine dependent inactivation of tissue plasminogen activator by a class PAI-2 inhibitor (minactivin).

Two-chain tissue plasminogen activator (t-PA) was found to be inactive in a coupled colorimetric assay for plasminogen activators, but a high level of activity was obtained in the presence of poly-D-lysine. This stimulated activity was strongly inhibited by minactivin, a urokinase inhibitor, but unstimulated enzyme could be shown to be unaffected by minactivin. In the presence of poly-D-lysine minactivin was a very successful competitive inhibitor of t-PA with respect to the substrate, plasminogen. The Ki for minactivin determined by the Henderson method was 2.5 X 10(-12) M, compared to the Km for plasminogen determined as 0.6 X 10(-6) M. The value of Ki for minactivin with u-PA, determined under the same conditions, was 1.6 X 10(-11) M.

Drug Interactions↗