Chemical modification of sarcoplasmic reticulum. Stimulation of Ca2+ release. (41/78)

Pretreatment of sarcoplasmic membranes with acetic or maleic anhydrides, which interact principally with amino groups, resulted in an inhibition of Ca2+ accumulation and ATPase activity. The presence of ATP, ADP or adenosine 5'-[beta, gamma-imido]triphosphate in the modification medium selectively protected against the inactivation of ATPase activity by the anhydride but did not protect against the inhibition of Ca2+ accumulation. Acetic anhydride modification in the presence of ATP appeared to increase specifically the permeability of the sarcoplasmic reticulum membrane to Ca2+ but not to sucrose, Tris, Na+ or Pi. The chemical modification stimulated a rapid release of Ca2+ from sarcoplasmic reticulum vesicles passively or actively loaded with calcium, from liposomes reconstituted with the partially purified ATPase fraction but not from those reconstituted with the purified ATPase. The inactivation of Ca2+ accumulation by acetic anhydride (in the presence of ATP) was rapid and strongly pH-dependent with an estimated pK value above 8.3 for the reactive group(s). The negatively charged reagents pyridoxal 5-phosphate and trinitrobenzene-sulphonate, which also interact with amino groups, did not stimulate Ca2+ release. Since these reagents do not penetrate the sarcoplasmic reticulum membranes, it is proposed that Ca2+ release is promoted by modification of internally located, positively charged amino group(s).  (+info)

Dicyclohexylcarbodiimide interaction with sarcoplasmic reticulum. Inhibition of Ca2+ efflux. (42/78)

Dicyclohexylcarbodiimide (DCCD), a hydrophobic carboxyl reagent, inhibited Ca2+ release from Ca2+-loaded sarcoplasmic reticulum vesicles, induced by elevated pH, tetraphenylboron, ATP + Pi, or membrane modification with acetic anhydride. Under the conditions used, the same concentrations of DCCD were required for inhibition of Ca2+ release, Ca2+-ATPase activity, and Ca2+ uptake. On the other hand, free Ca2+ or alkaline pH prevented the inhibition by DCCD of Ca2+-ATPase and coupled Ca2+ transport but not that of Ca2+ release. Moreover, several hydrophilic carboxyl reagents inhibited Ca2+-ATPase but not Ca2+ release. We suggest that a carboxyl residue(s), located in a hydrophobic region of a protein(s), is involved in the control of Ca2+ release, where DCCD interaction with this group blocks Ca2+ release. This group is distinct from the one involved in the inhibition of Ca2+-ATPase. DCCD also inhibited [3H]ryanodine binding to junctional sarcoplasmic reticulum membranes. The presence of Ca2+ or an alkaline pH only slightly affects the degree of inhibition of ryanodine binding by DCCD. Incubation of the membranes with [14C]DCCD resulted in labeling of 350-, 170-, 140-, 53-, and 30-kDa proteins in addition to the Ca2+-ATPase. The involvement of one or all of the DCCD-labeled proteins in Ca2+ release and ryanodine binding is discussed.  (+info)

Regeneration of native bacteriorhodopsin structure following acetylation of epsilon-amino groups of Lys-30, -40, and -41. (43/78)

The chymotryptic fragment of bacteriorhodopsin, C-2 (residues 1-71), has been acetylated completely at its three lysines (residues 30, 40, and 41) by treatment with acetic anhydride. The triacetylated C-2 fragment is able to reassociate with fragment C-1 (residues 72-248) and the complex binds all-trans-retinal to form a native bacteriorhodopsin-like chromophore, which is essentially identical with that formed from fragments C-2 and C-1. Further, the kinetics and pH dependence of chromophore regeneration and the proton pumping of the reconstituted triacetylated C-2 and C-1 complex are indistinguishable from that of the unmodified C-2 and C-1 complex. However, the extent of regeneration of the chromophore from triacetylated C-2 and C-1 is less than that from fragments C-2 and C-1, suggesting that the acetylated C-2 fragment is less stable than unacetylated C-2 in the reconstitution medium. We conclude that the amino groups in Lys-30, -40, and -41 do not contribute to the stabilization of the folded bacteriorhodopsin structure and are not required for proton translocation.  (+info)

Evidence that the cytoplasmic aldehyde dehydrogenase-catalysed oxidation of aldehydes involves a different active-site group from that which catalyses the hydrolysis of 4-nitrophenyl acetate. (44/78)

Acylation of the aldehyde dehydrogenase.NADH complex by acetic anhydride leads to the production of acetaldehyde and NAD+. By monitoring changes in nucleotide fluorescence, the rate constant for acylation of the active site of the *enzyme.NADH complex was found to be 11 +/- 3 s-1. The rate of acylation by acetic anhydride at the group that binds aldehydes on the oxidative pathway is clearly rapid enough to maintain significant steady-state concentrations of the required active-site-acylated *enzyme.NADH intermediate despite the rapid hydrolysis of this *enzyme.acyl.NADH intermediate (5-10 s-1) [Blackwell, Motion, MacGibbon, Hardman & Buckley (1987) Biochem. J. 242, 803-808]. Hence reversal of the normal oxidative pathway can occur. However, although acylation of the aldehyde dehydrogenase.NADH complex by 4-nitrophenyl acetate also occurs rapidly with a rate constant of 10.9 +/- 0.6 s-1, even under the most extreme trapping conditions only very small amounts of acetaldehyde are detected [Loomes & Kitson (1986) Biochem. J. 235, 617-619]. Furthermore enzyme-catalysed hydrolysis of 4-nitrophenyl acetate is limited by the rate of deacylation of a group on the enzyme (0.4 s-1), which is an order of magnitude less than deacylation of the group at the active site (5-10 s-1). It is concluded that the enzyme-catalysed 4-nitrophenyl ester hydrolysis involves a group on the enzyme that is different from the active-site group that binds aldehydes on the normal oxidative pathway.  (+info)

Iodination-induced alterations in biochemical properties of human placental insulin receptor. (45/78)

Insulin receptors from human placenta have been labeled by using an oxidative iodination procedure (iodogen-mediated or chloramine-T-mediated), Bolton-Hunter reagent or [3H]acetic anhydride. The oxidative iodination procedure reduces the affinity for 131I-insulin and the receptor protein becomes fragmented into smaller pieces with an s20,w value of 5-6. However, treatment with Bolton-Hunter reagent or [3H]acetic anhydride does not alter the Kd of 131I-insulin binding and the s20,w value remains unchanged with respect to the native receptor. It is proposed that for labeling multisubunit sulfhydryl-linked protein drastic oxidative iodination procedures should be avoided.  (+info)

Pitfalls in the use of carboxylic acid anhydrides for structural studies of nucleoprotein particles. (46/78)

During modification of protein amino groups with carboxylic acid anhydrides, these reagents cause a fall in pH, which can be prevented by addition of base. Although unmodified nucleosomal particles are not affected by the local transient changes in pH induced by the base (NaOH) added to prevent a fall in pH during modification, the nucleosomal particles modified by acetic anhydride are dissociated, with release of single-stranded DNA.  (+info)

Aspirin treatment reduces platelet resistance to deformation. (47/78)

The present investigation has evaluated the influence of aspirin, its constituents, and other nonsteroidal anti-inflammatory agents on the resistance of human platelets to aspiration into micropipettes. Aspirin increased the length of platelet extensions into the micropipette over the entire negative tension range of 0.04 to 0.40 dynes/cm after exposure to the drug in vitro or after ingestion of the agent. Other cyclooxygenase inhibitors, ibuprofen and indomethacin, did not increase platelet deformability. The influence of aspirin was mimicked to some degree by high concentrations of salicylic acid, but acetylation of platelets with acetic anhydride had little influence on platelet deformability. Incubation of platelets with both salicylic acid and acetic anhydride had no more effect than salicylic acid alone. Benzoic acid, chemically similar to salicylic acid, had a minimal effect. The studies demonstrate that aspirin makes platelets more deformable, while components of the drug or other nonsteroidal antiinflammatory agents and cyclooxygenase inhibitors do not have the same influence on resistance to deformation.  (+info)

Lipoproteins of the extravascular space: enhanced macrophage degradation of low density lipoproteins from interstitial inflammatory fluid. (48/78)

Current evidence has demonstrated that cholesteryl ester-loaded macrophages are important components of the atherosclerotic lesion. Additional studies have implicated low density lipoproteins (LDL) and circulating monocytes as central to the origin of lipid-laden foam cells found in the arterial wall. This is a result of the finding of accelerated macrophage uptake of LDL chemically modified by reaction with malondialdehyde (MDA-LDL), acetic anhydride (Ac-LDL), or incubation with arterial cells in vitro. In concert with these chemical modifications, we have previously demonstrated selective in vivo modification of LDL isolated from interstitial inflammatory fluid (IF) of the rabbit. Utilizing the polyvinyl sponge implant model, we reported that IF-LDL had an altered chemical composition, electrophoretic mobility, and particle size distribution when compared to LDL isolated from homologous plasma (WP-LDL). In this study reported herein, we examined the metabolism of IF-LDL by resident mouse peritoneal macrophages (MPM) in culture. IF-LDL was degraded substantially faster by MPM, and resulted in a substantial increase in cellular cholesteryl ester when compared to cells incubated with WP-LDL. IF-LDL binding to MPM was inhibited by Ac-LDL derived from WP-LDL, but only minimally by unmodified WP-LDL. Transmission electron microscopy of MPM revealed extensive lipid deposition in cells incubated with Ac-LDL and IF-LDL. These results implicate LDL from interstitial inflammatory fluid as an in vivo modified lipoprotein that can enhance uptake via the acetyl LDL receptor pathway in resident macrophages.  (+info)