J.David Johnson

Hydrolysis of a fluorescent phospholipid substrate by phospholipase A2 and lipoprotein lipase.

The fluorescent phospholipid 1-acyl-2-[6-[(7-nitro-2,1,3benzoxadiazol-4 -yl) amino]-caproyl] phosphatidylcholine (C6-NBD-PC) was used as a substrate for porcine pancreatic phospholipase A2 (PA2) and bovine milk lipoprotein lipase (LpL). Hydrolysis of C6-NBD-PC by either enzyme resulted in a greater than 50-fold fluorescence enhancement with no shift in the emission maximum at 540 nm; Ca++ was required for PA2 catalysis. Identification of the products of hydrolysis showed cleavage at the sn-1 and sn-2 positions for LpL and PA2, respectively. For PA2, but not for LpL, there was a marked enhancement of enzyme catalysis at lipid concentrations above the critical micellar concentration of the lipid. Furthermore, apolipoprotein C-II, the activator protein of LpL for long-chain fatty acyl substrates, did not enhance the rate of catalysis of the water-soluble fluorescent phospholipid for either enzyme.

Allosteric interactions among drug binding sites on calmodulin

Felodipine is a fluorescent dihydropyridine Ca2+-antagonist. It binds to calmodulin in a Ca2+-dependent manner, and undergoes a fluorescence increase which allows us to monitor its interaction with calmodulin. Hydrophobic ligands including the calmodulin antagonist, R24571 and Ca2+ antagonists, prenylamine and diltiazem, bind to calmodulin and potentiate felodipine binding by as much as 20 fold. These studies suggest that allosteric interactions occur among different drug binding sites on calmodulin. Our results are discussed in terms of the mechanism of action of calmodulin.

Lipoprotein lipase- and phospholipase A2-catalyzed hydrolysis of phospholipid vesicles with an encapsulated fluorescent dye. Effects of apolipoproteins.

The self-quenching dye, 6-carboxyfluorescein, has been encapsulated into sonicated vesicles of egg phosphatidylcholine. Porcine pancreatic phospholipase A2 and bovine milk lipoprotein lipase catalyze the hydrolysis of the phosphatidylcholine resulting in the release of the encapsulated dye and a large increase in 6-carboxyfluorescein fluorescence. The fluorescence increase occurs in parallel with the formation of lysophosphatidylcholine and is strongly dependent on Ca2+ for phospholipase A2 catalysis and on apolipoprotein C-II for hydrolysis by lipoprotein lipase. Other apolipoproteins, including apolipoproteins C-III, C-I, and A-I, do not enhance lipoprotein lipase activity towards this substrate. We conclude that the enhancement of lipoprotein lipase activity by apolipoprotein C-II is a specific property of the activator protein due to its interaction with lipoprotein lipase or an enzyme/lipid interface and not a characteristic of lipid-binding proteins in general.

Immunological studies on bovine milk lipoprotein lipase. Effects of Fab fragments on enzyme activity.

Rabbit antiserum was prepared against purified bovine mild lipoprotein lipase. Immunoelectrophoresis of lipoprotein lipase gave a single precipitin line against the antibody which was coincident with enzyme activity. The gamma-globulin fraction inhibited heparin-releasable lipoprotein lipase activity of bovine arterial intima, heart muscle and adipose tissue. The antibody also inhibited the lipoprotein lipase activity from adipose tissue of human and pig, but not that of rat and dog. Fab fragments were prepared by papain digestion of the gamma-globulin fraction. Fab fragments inhibited the lipoprotein lipase-catalyzed hydrolysis of dimyristoylphosphatidylcholine vesicles and trioleoylglycerol emulsions to the same extent. The Fab fragments also inhibited the lipolysis of human plasma very low density lipoproteins. The change of the kinetic parameters for the lipoprotein lipase-catalyzed hydrolysis of trioleoylglycerol by the Fab fragments was accompanied with a 3-fold increase in Km and a 10-fold decrease in Vmax. Preincubation of lipoprotein lipase with apolipoprotein C-II, the activator protein for lipoprotein lipase, did not prevent inhibition of enzyme activity by the Fab fragments. However, preincubation with dipalmitoylphosphatidylcholine-emulsified trioleoylglycerol or Triton X-100-emulsified trioleoylglycerol had a protective effect (remaining activity 7.0 or 25.8%, respectively, compared to 1.0 or 0.4% with no preincubation). The addition of both apolipoprotein C-II and substrate prior to the incubation with the Fab fragments was associated with an increased protective effect against inhibition of enzyme activity; remaining activity with dipalmitoylphosphatidylcholine-emulsified trioleoylglycerol was 40.6% and with Triton X-100-emulsified trioleoylglycerol, 45.4%. Human plasma very low density lipoproteins also protected against the inhibition of enzyme activity by the Fab fragments. These immunological studies suggest that the interaction of lipoprotein lipase with apolipoprotein C-II in the presence of lipids is associated with a conformational change in the structure of the enzyme such that the Fab fragments are less inhibitory. The consequence of a conformational change in lipoprotein lipase may be to facilitate the formation of an enzyme-triacylglycerol complex so as to enhance the rate of the lipoprotein lipase-catalyzed turnover of substrate to products.

Incorporation of dipalmtoyl phosphatidylcholine into human plasma low density lipoproteins: Effects on composition and structure

Bovine liver phosphatidylcholine exchange protein was used to transfer di[14C]palmitoyl phosphatidylcholine from sonicated vesicles to human plasma low density lipoproteins (LDL). The incorporation of di[14C]-palmitoyl phosphatidylcholine into LDL was associated with an increased percentage of total phospholipid and increased lipid rigidity as shown by an increase in the fluorescence polarization of 12-(9-anthroyloxy)-stearic acid and 1,6-diphenyl-1,3,5-hexatriene.

The Regulation of Cardiac-Muscle Contraction by Troponin

The purpose of this review is to summarize the work we have done over the past several years regarding the regulation of cardiac muscle contraction by troponin (Tn). Studies have been carried out on the Ca2+-binding properties of cardiac Tn (the 1:1:1 molar-ratio complex of TnC, the Ca2+-binding subunit; TnI, the inhibitory subunit; and TnT, the tropomyosin-binding subunit) and TnC, and the effects of Mg2+ and phosphorylation on these parameters have been measured. The relationship between these parameters and the activation of myofibrillar ATPase in cardiac and skeletal muscle will be discussed and related to the known physiological properties of cardiac muscle in different contractile states (e.g., ³-adrenergic stimulation). Since the binding of Ca2+ to Tn is only the first step in the activation of muscle contraction, studies on Ca2+-induced alterations in the interactions of Tn subunits, tropomyosin (Tm), and actin required to bring about muscle contraction or relaxation will also be discussed.

Mutation of highly conserved arginine residues disrupts the Structure and Function of Annexin V

BACKGROUND Annexins are a family of structurally related proteins that bind to phospholipid membranes in a Ca(2+)-dependent manner. Annexins are characterized by highly conserved canonical domains of approximately 70 amino acids. Annexin V contains four such domains. Each of these domains has a highly conserved arginine (R). METHODS To evaluate the role of the conserved arginines in the molecular structure of annexin V, negatively charged amino acids were substituted for arginines at positions R43, R115, R199, and R274 using site-directed mutagenesis. RESULTS Mutants R199D and R274E were rapidly degraded when expressed in bacteria, and were not further characterized. R43E exhibited an electrophoretic mobility similar to the wild-type protein, while R115E migrated significantly in a slower fashion, suggesting a less compact conformation. R43E and R115E exhibited much greater susceptibility to proteolytic digestion than the wild type. While Ca(2+)-dependence for phospholipid binding was similar in both mutants (half-maximal 50-80 microM Ca2+), R43E and R115E exhibited a 6- and 2-fold decrease in phospholipid affinity, respectively. Consistent with the different phospholipid affinities of the annexins, a phospholipid-dependent clotting reaction, the activated partial thromboplastin time (aPTT), was significantly prolonged by the wild-type protein and mutants R115E and R115A. The aPTT was unaffected by R43E. CONCLUSIONS Our data suggest that mutation of these highly conserved arginine residues in each of the four canonical domains of annexin have differential effects on the phospholipid binding, tertiary structure, and proteolytic susceptibility of annexin V. The site I mutation, R43E, produced a large decrease in phospholipid affinity associated with an increase in proteolytic susceptibility. The site II mutation, R115E, produced a small change in phospholipid binding but a significant modification of electrophoretic mobility. Our data suggest that highly conserved arginine residues are required to stabilize the tertiary structure of annexin V by establishing hydrogen bonds and ionic bridges.

Lipoprotein Structure and the Mechanism of Action of Lipoprotein Lipase

Circulating lipoprotein triglyceride is cleared in extrahepatic tissues by lipoprotein lipase (LpL). The enzyme is normally attached to the capillary endothelium, but is released into the plasma by the injection of heparin. LpL has been purified and characterized from post-heparin plasma, from tissues and from milk (Augustin and Greten 1979). The characteristic feature of LpL is that it is stimulated by apolipoprotein C-II (apoC-II), a protein (Jackson et al. 1977) present in chylomicrons, very low density lipoproteins (VLDL) and high density lipoproteins (HDL). This report describes the role of apoC-II and lipid structure in the mechanism of action of bovine milk lipoprotein lipase.