Inhibition of rabbit lung angiotensin-converting enzyme by N alpha-[(S)-1-carboxy-3-phenylpropyl]L-alanyl-L-proline and N alpha-[(S)-1-carboxy-3-phenylpropyl]L-lysyl-L-proline

Two novel peptide analogs, N alpha-[(S)-1-carboxy-3-phenylpropyl]L-alanyl-L-proline and the corresponding L-lysyl-L-proline derivative, have been demonstrated to be potent competitive inhibitors of purified rabbit lung angiotensin-converting enzyme: Ki = 2 and 1 X 10(-10) M, respectively, at pH 7.5, 25 degrees C, and 0.3 M chloride ion. Second-order rate constants for addition of these inhibitors to enzyme under the same conditions are in the range 1-2 X 10(6) M-1 s-1; first-order rate constants for dissociation of the EI complexes are in the range 1-4 X 10(-4) s-1. The association rate constants are similar to those measured for D-3-mercapto-2-methylpropanoyl-L-proline, captopril, but the dissociation rate constants are severalfold slower and account for the higher affinity of these inhibitors for the enzyme. The dissociation constant for the EI complex containing N alpha-[(S)-1-carboxy-3-phenylpropyl]L-alanyl-L-proline is pH-dependent, and reaches a minimum at approximately pH 6: Ki = 4 +/- 1 X 10(-11) M. The pH dependence is consistent either with a model for which the protonation state of the secondary nitrogen atom in the inhibitor determines binding affinity, or one for which ionizations on the enzyme alone influence affinity for these inhibitors. The affinity of this inhibitor for the zinc-free apoenzyme is 2 X 10(4) times less than for the zinc-free apoenzyme is 2 X 10(4) times less than that for the holoenzyme. If considered as a "collected product" inhibitor, N alpha-[(S)-1-carboxy-3-phenylpropyl]L-alanyl-L-proline appears to derive an additional factor of 375 M in its affinity for the enzyme compared to that of the two products of its hypothetical hydrolysis, a consequence of favorable entropy effects.     

Interaction of human plasma low density lipoprotein with concanavalin A and with ricin.

Native human plasma low density lipoprotein (LDL) interacts with concanavalin A but not with ricin; apOLDL reacts with both lectins. Each reaction is inhibited by the appropriate lectin-specific carbohydrate. The "receptors" on LDL for these two lectins are not destroyed by digestion by proteolytic enzymes. Peptide hydrolysis does not influence the reactivity of LDL toward concanavalin A. It does, however, substantially enhance the ability of the lipoprotein to interact with ricin. The data strongly suggest that the carbohydrate protion of a glycoprotein component of LDL is bound at the saccharidespecific active site on the lectin.     

Phospholipases. III. Effects of ionic surfactants on the phospholipase-catalyzed hydrolysis of unsonicated egg lecithin liposomes.

Apparent values of Km and Vmax have been measured for catalysis of hydrolysis of unsonicated egg lecithin liposomes, activated through addition of 0.4 M n-hexanol, by phospholipases A2 from bee and snake venoms and by phospholipase C from Clostridium welchii as a function of the concentration of three surfactants: hexadecylamine, hexadecyltrimethylammonium bromide, and dihexadecyl phosphate. For all three enzymes, values of Km and Vmax show little or no dependence on the concentration of these ionic surfactants, demonstrating that the liposomal surface charge is not a crucial factor in determining susceptibility to phospholipase-catalyzed hydrolysis.     

Context-Dependent Sensitivity to Mutations Disrupting the Structural Integrity of Individual EGF Repeats in the Mouse Notch Ligand DLL1

The highly conserved Notch-signaling pathway mediates cell-to-cell communication and is pivotal for multiple developmental processes and tissue homeostasis in adult organisms. Notch receptors and their ligands are transmembrane proteins with multiple epidermal-growth-factor-like (EGF) repeats in their extracellular domains. In vitro the EGF repeats of mammalian ligands that are essential for Notch activation have been defined. However, in vivo the significance of the structural integrity of each EGF repeat in the ligand ectodomain for ligand function is still unclear. Here, we analyzed the mouse Notch ligand DLL1. We expressed DLL1 proteins with mutations disrupting disulfide bridges in each individual EGF repeat from single-copy transgenes in the HPRT locus of embryonic stem cells. In Notch transactivation assays all mutations impinged on DLL1 function and affected both NOTCH1 and NOTCH2 receptors similarly. An allelic series in mice that carried the same point mutations in endogenous Dll1, generated using a mini-gene strategy, showed that early developmental processes depending on DLL1-mediated NOTCH activation were differently sensitive to mutation of individual EGF repeats in DLL1. Notably, some mutations affected only somite patterning and resulted in vertebral column defects resembling spondylocostal dysostosis. In conclusion, the structural integrity of each individual EGF repeat in the extracellular domain of DLL1 is necessary for full DLL1 activity, and certain mutations in Dll1 might contribute to spondylocostal dysostosis in humans.     

Identification of the hemoglobin binding sites on the inner surface of the erythrocyte membrane

The hemoglobin binding sites on the inner surface of the erythrocyte membrane were identified by measuring the fraction of hemoglobin released following selective proteolytic or lipolytic enzyme digestion. In addition, binding stoichiometry to and fractional hemoglobin release from inside-out vesicle preparations of human and rabbit membranes were compared since rabbit membranes differ significantly from human membranes only in that they lack glycophorin. Our results show that rabbit inside-out vesicles bind about 65% less human or rabbit hemoglobin under conditions of optimal and stoichiometric binding, despite being otherwise similar in composition. We suggest that this difference is either directly or indirectly due to the absence of glycophorin in rabbit membranes. Further supportive evidence includes demonstrating (a) that neuraminidase treatment of human membranes did not affect hemoglobin binding and (b) that reconstitution of isolated glycophorin into phospholipid vesicles increased the hemoglobin binding capacity in a manner proportional to the fraction of glycophorin molecules oriented with their cytoplasmic sides exposed to the exterior of the vesicle. Proteolysis of human inside-out vesicles either before or after addition of hemoglobin reduced the binding capacity by about 25%. This is consistent with the known proportion of total hemoglobin binding sites involving band 3 protein and the selective lability of the cytoplasmic aspect of band 3 protein to proteolysis. Phospholipid involvement in hemoglobin binding was determined using various phospholipase C preparations which differ in their reactivity profiles. Approximately 38% of the bound hemoglobin was released upon cleavage of phospholipid headgroups. These results suggest that the predominant sites of binding for hemoglobin on the inner surface of the red cell membrane are the two major integral membrane glycoproteins.