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Biomedical subjects

F Hofmann

Publications and source records attributed to F Hofmann.

At least 379 records · Page 21Linked to original sources
PubMed1977

Concentrations of cyclic AMP-dependent protein kinase subunits in various tissues.

The concentrations of the regulatory (R) and catalytic (C) subunits of adenosine 3':5'-monophosphate (cyclic AMP)-dependent protein kinase(s) were measured in extracts of skeletal muscle, heart, liver, kidney, and brain. These concentrations were also estimated for the particulate fraction from brain, the only tissue in which a major part of the total activity was not readily extracted in a soluble form. Values for R were determined by measuring the amount of cyclic [3H]amp bound to protein in these tissue fractions under specified conditions; it was assumed that 1 mol of cyclic AMP binds to 1 mol of R. Values for C were determined from measurements of the specific protein kinase activity of the fractions utilizing the turnover number of pure C in the calculations. Turnover numbers for C were found to be identical for this subunit obtained in the pure form from rabbit skeletal muscle, rabbit liver, and beef heart. The methods used for measuring C were evaluated by kinetic studies and through the use of the specific heatstable protein inhibitor of cyclic AMP-dependent protein kinase(s). R and C were found to exist in a 1:1 molar ratio in all of the tissue fractions that were studied. the absolute concentrations of R and C ranged from 0.23 mumol/kg wet weight for liver to 0.78 mumol/kg wet weight for brain. For brain this value was based on the amount of each subunit in the particulate as well as the soluble fraction. For other tissues the values were based solely on the subunit content of the latter fraction. It was noted that the molar concentrations of R are close to those of cyclic AMP under basal conditions in the various tissues.

Animals↗
PubMed1976

Response of isolated rat hepatocytes to D-galactosamine and uridine.

Isolated rat hepatocytes preserved for several hours their contents of ATP, CTP, GTP, UTP and UDPG. Leakage of intracellular and membrane-bound enzymes remained in the range observed with the isolated perfused rat liver. D-Galactosamine led to a rapid decrease of the UTP and UDPG contents while ATP and GTP levels remained unaffected. The sum of acid-soluble uracil nucleotides increased immediately after the addition of D-galactosamine, indicating release of the UTP-mediated feed-back inhibition of pyrimidine nucleotide de novo biosynthesis. Addition of uridine to a hepatocyte suspension resulted in a rapid increase in the levels of uridylate derivatives; the concentrations of UTP and UDPG, previously depleted by D-galactosamine, were restored within 1 h. Isolated hepatocytes appear suitable for studies on the pathogenic sequence of events elicited by D-galactosamine and on the regulation of pyrimidine biosynthesis.

Adenosine Triphosphate↗
PubMed1976

Effect of d-galactosamine administration on nucleotide and protein metabolism in isolated rat Kupffer cells.

The nucleoti-e contents of isolated rat Kupffer cells were found to be smaller than those of hepatocytes. The rate of UDPGal formation from D-galactose was much lower in Kupffer cells than in hepatocytes. The viability of the former was checked by measuring the leakage of enzymes and the formation of UTP from uridine. Addition of GalN to isolated Kupffer cells did not decrease their UTP and UDPG contents as much as those of hepatocytes. The same results were obtained when cells were isolated from GalN-pretreated animals. The incorporation of labeled amino acids into protein after GalN addition was much less reduced in Kupffer cells than in hepatocytes. The data suggest that Kupffer cells do not contribute to GalN-induced liver injury as a result of uridylate trapping.

Amino Acids↗
PubMed1975

Comparison of adenosine 3':5'-monophosphate-dependent protein kinases from rabbit skeletal and bovine heart muscle.

Homogeneous preparations of adenosine 3':5'-monophosphate (cyclic AMP)-dependent protein kinase from rabbit skeletal (Peak I) and bovine heart muscle have been compared. Each enzyme has an S20,w value of 7.0. Each enzyme binds 2 mol of cyclic AMP per mol of enzyme and is dissociated in the presence of saturating concentrations of cyclic AMP into a demeric regulatory subunit-cyclic AMP complex and two catalytic subunits. The isolated subunits recombine, resulting in the formation of the original holoenzyme in each case. Several differences between the two enzymes were found. Different salt concentrations are necessary for elution of the respective enzyme from DEAE-cellulose. Their regulatory subunits differ with respect to their sedimentation constants and mobility on sodium dodecyl sulfate gel electrophoresis. The regulatory subunit of the heart enzyme is rapidly phosphorylated by MgATP but this does not occur with the skeletal muscle enzyme. MgATP is bound with high affinity only to the skeletal muscle enzyme. The enzymes have different apparent dissociation constants and Hill coefficients for cyclic AMP binding. With the skeletal muscle enzyme MgATP increases the dissociation constants for cyclic AMP about 10-fold and decreases the Hill coefficient, while with the heart enzyme phosphorylation decreases the cissociation constant for cyclic AMP 5- to 6-fold and increases the Hill coefficient. Different concentrations of cyclic AMP are required to dissociate the skeletal and heart muscle enzymes. The presence of MgATP increases the concentration of cyclic AMP required to dissociate the skeletal muscle enzyme but decreases the concentration necessary to dissociate the heart enzyme.

Adenosine Triphosphate↗
PubMed1975

Reversibility of adenosine 3':5'-monophosphate-dependent protein kinase reactions.

Using a homogeneous enzyme from rabbit skeletal muscle, it has been demonstrated that the cyclic adenosine 3':5'-monophosphate (cyclic AMP)-dependent protein kinase reaction is reversible. In addition to the phosphorylated protein substrate, the reverse reaction requires Mg2+, ADP, and cyclic AMP when the holoenzyme is used as the source of enzyme. It is independent of cyclic AMP when the catalytic subunit of the protein kinase is used. The optimum pH for the reverse reaction with 32P-labeled casein as the substrate is 5.7, essentially the same as that for the forward reaction. Among the nucleotide subtrates tested, ADP serves as the best phosphoryl group acceptor. The Km of the enzyme for ADP is 3.3 mM and that for 32P-casein is 1.7 mg/ml. The equilibrium constant at 30 degrees is approximately 0.042 at a magnesium concentration of 10 mM and a pH of 6.9. This result indicates that the free energy of hydrolysis (deltaG0obs) of the phosphorylated protein substrate is relatively high, i.e. approximately -6.5 kcal/mol under these conditions.

Adenosine Diphosphate↗