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M Landt

Publications and source records attributed to M Landt.

102 records · Page 6Linked to original sources
PubMed1982

A survey of calmodulin-activated protein kinase activity in several tissues of Rattus rattus.

1. In each of five tissues (brain, heart, spleen, mammary acini and pancreatic acini) the prominent endogenous protein substrate possessed a molecular weight between 51,900 and 56,800 on SDS gel electrophoresis. 2. Evidence was obtained for two species of protein substrate, differing slightly in molecular weight, which appear to be distributed in a tissue-specific manner. 3. One species, with a molecular weight of 52,900, was found in spleen, mammary acini and brain; the other species, with a molecular weight of 56,200, was found in heart and pancreatic acini. 4. The specific activity of calmodulin-activated protein kinase in homogenates varied from a high of 44 pmol/min/mg in brain to 4.2 pmol/min/mg in mammary acini. 5. Subcellular fractionation of these tissues demonstrated that most of the activity was found in the cytosolic fraction and a "light-particle" fraction obtained by ultracentrifugation, but the kinase was not associated with endoplasmic reticulum. 6. High concentrations of calmodulin were required to activate the protein kinase activity from each tissue. 7. Calmodulin concentrations producing half-maximal activation were 94 nM for brain, 377 nM for spleen, 132 nM for pancreatic acini, 350 nM for heart, and 117 nM for mammary acini. 8. The calmodulin-activated protein kinase activity in these tissues were similar, but the few differences in properties from tissue to tissue left open the possibility that multiple, calmodulin-activated kinase activities exist in these tissues.

Animals↗
PubMed1980

Ca2+-activated ATPase and ATP-dependent calmodulin-stimulated Ca2+ transport in islet cell plasma membrane.

Calcium is known to play an essential part in the regulation of insulin secretion in the pancreatic beta cell. Calcium influx/efflux studies indicate that glucose promotes an accumulation of calcium by the beta cell. However, interpretation of such data is particularly difficult due to the complex compartmentalization of calcium within the cell. Although indirect evidence using chlorotetracycline suggests that control of calcium homeostasis at the plasma membrane may be central to insulin secretion, the mechanism by which secretagogues influence the handling of calcium remains unknown. Despite its continuous diffusive entry, intracellular calcium is maintained in the submicromolar range by energy-dependent mechanisms. One such process which has been well characterized in erythrocytes is a plasma membrane calcium extrusion pump whose enzymatic basis is a high affinity (Ca+2 + Mg+2)ATPase. A similar mechanism regulated by insulin has recently been identified in adipocyte plasma membranes. We report here the presence of a high affinity (Ca+2 + Mg+2)ATPase and ATP-dependent calmodulin-stimulated calcium transport system in rat pancreatic islet cell plasma membranes.

Adenosine Triphosphate↗
PubMed1980

Calmodulin-sensitive ATP-dependent Ca2+ transport across adipocyte plasma membranes.

An ATP-dependent transport system which is active at concentrations of free Ca2+ in the submicromolar range has been identified in adipocyte plasma membranes. The system appears to represent the functional component of the high affinity insulin-sensitive calcium-stimulated, magnesium-dependent adenosine triphosphatase preveiously described in the same preparation (Pershadsingh, H. A., and McDonald, J. M. (1979) Nature 281, 495-497). This ATP-dependent Ca2+ transport pump was stimulated approximately 3-fold by the Ca2+-dependent regulatory protein, calmodulin. This effect was confined to the plasma membrane since a similar effect was undetectable in the fraction enriched in endoplasmic reticulum. Calmodulin stimulation was dose-dependent but saturable with half-maximal activation occurring at 0.72 microgram/ml (43 nM). Calmodulin appeared to stimulate the system primarily by decreasing the apparent half-maximal saturation constant for free Ca2+ from 0.20 +/- 0.04 microM to 0.07 +/- 0.01 microM (n = 3). The Hill coefficient increased from 1.6 +/- 0.2 to 3.2 +/- 0.6 (n = 3), thus showing an increased positive cooperativity which allows the pump to be activated by an exceedingly narrow Ca2+ threshold in the presence of calmodulin. The calmodulin stimulation of the plasma membrane Ca2+ extrusion pump in adipocytes, working in opposition to metabolic signals which increase cytoplasmic Ca2+, could constitute a self-regulating negative feedback device for maintaining a low steady state level of intracellular Ca2+. This feedback system may be of critical importance in regulation of cellular metabolism by insulin.

Adipose Tissue↗
PubMed1980

5'-Nucleotide phosphodiesterase: features of the substrate binding site as deduced from specificity and kinetics of some novel substrates.

Phosphonate monoesters and phosphate diesters with systematically varied substituents and leaving groups were synthesized and tested as substrates for homogeneous 5'-nucleotide phosphodiesterase from bovine intestine. The enzyme was shown to hydrolyze phosphorothioate and phosphonoamidate compounds but at significantly lower rates than comparable oxy compounds. The effects of bulk and structure of the ester or phosphonate substituents were also investigated. Dibenzyl phosphate, an ester of an aliphatic alcohol, was a poor substrate. The enzyme did not hydrolyze aliphatic monoesters of phosphonates, regardless of bulk. Kinetic parameters of several nitrophenyl phosphonomonoesters and phosphodiesters are presented. The results suggest that synthetic nonnucleotide substrates can bind in two different modes, only one of which is productive. Incidence of nonproductive binding, with consequent kinetic effects, is increased by increasing the symmetry of the substrates.

Animals↗
PubMed1978

Alkaline phosphatase: affinity chromatography and inhibition by phosphonic acids.

Five phosphonic acid derivatives were synthesized, coupled to agarose, and tested for affinity chromatographic binding of alkaline phosphatase from bovine intestine. Agarose coupled to L-histidyldiazobenzylphosphonic acid was found to be a highly effective adsorbent. In order to understand the large differences in binding capacity observed with derivatized agaroses, inhibition of alkaline phosphatase by phosphonic acid ligands, and related phosphonic acids, was measured. The results of affinity chromatography and inhibition studies were in good agreement, demonstrating that phosphonic acids with large aromatic/hydrophobic, carboxylate substituents bind strongly and competitively to the enzyme active site.

Alkaline Phosphatase↗
PubMedDate not supplied

Hyperleptinemia in patients with end-stage renal disease undergoing continuous ambulatory peritoneal dialysis.

OBJECTIVE: To determine whether the increased plasma leptin levels reported in hemodialyzed patients is a feature of end-stage renal disease or an artifact of hemodialysis, we studied plasma levels in patients treated exclusively by continuous ambulatory peritoneal dialysis (CAPD). DESIGN: Prospective comparison of end points in CAPD patients and matched healthy subjects. SETTING: Tertiary care institutional dialysis center. PARTICIPANTS: Fifty-six healthy subjects, age 50.8 +/- 2.3 years, body mass index (BMI) 27.7 +/- 1.3 kg/m2, recruited through public announcement, and 36 patients with end-stage renal disease, age 51.0 +/- 2.4 yr, BMI 28.2 +/- 1.3 kg/m2, enrolled in a CAPD treatment program. INTERVENTION: Four exchanges of CAPD per day, using 2.0, 2.5, or 3.0 L of dialysate over a period of 1 - 96 months (median 22 mth). MAIN OUTCOME MEASURES: The primary outcome measure was plasma leptin concentration. Secondary measures included plasma glucose, insulin, C-peptide, and cortisol concentrations; and residual renal function and dialysis adequacy (Kt/V). RESULTS: Plasma leptin levels in CAPD patients were 27.1 - 490 ng/mL (women) and 1.3 - 355 ng/mL (men); the levels in healthy subjects were 2.0 - 84.7 ng/mL (women) and 1.8 - 55.4 ng/mL (men). The mean leptin levels were 5-fold higher among CAPD-treated men than control men (49.9 +/- 18.4 vs 9.8 +/- 2.5 ng/mL, p < 0.001) and 7.5-fold higher among CAPD-treated women than control women (220 +/- 28.1 vs 29.3 +/- 3.7 ng/mL, p < 0.0001). Female gender and BMI were the strongest predictors of hyperleptinemia in CAPD patients. CONCLUSION: These results indicate that hyperleptinemia is a feature of terminal renal failure, not an artifact of hemodialysis.

Artifacts↗