Substrate analogue induced changes of the CO-stretching mode in the cytochrome P450cam-carbon monoxide complex.

The CO-stretching mode of the carbon monoxide ligand in reduced cytochrome P450cam, in the absence or presence of camphor and in the presence of nine different camphor analogues, was measured at room temperature using Fourier transform infrared spectroscopy. Substrate-free cytochrome P450cam--CO reveals a broad, slightly structured band resulting from an overlap of several stretching mode signals. The multitude of the signals indicates that cytochrome P450 exists in a dynamic equilibrium of several conformational substates. Binding of camphor or camphor analogues strongly influences this equilibrium. For substrate analogues which are not able to form a hydrogen bond to the hydroxyl group of tyrosine 96, the CO-stretching band is rather broad and asymmetric. In contrast, substrate analogues with one quinone group which form a hydrogen bond to the Tyr96 OH induce a shift and a sharpening of the CO-stretching mode band. For substrate analogues with two hetero groups, the infrared spectrum is slightly asymmetric or a minor band appears. Sterical hindrance, substrate mobility, and protein flexibility finally determine the position and width of the CO-stretching mode signals.     

Binding of myosin to actin in myofibrils during ATP hydrolysis

Measurements of cross-bridge attachment to actin in myofibrils during ATP hydrolysis require prior fixation of myofibrils to prevent their contraction. The optimal cross-linking of myofibrils was achieved by using 10 mM carbodiimide (EDC) under rigor conditions and at 4 degrees C. The fixed myofibrils had elevated MgATPase activity (150%) and could not contract. As judged by chymotryptic digestions and subsequent SDS gel electrophoresis analysis, less than 25% of myosin heads were cross-linked in these myofibrils. The isolated, un-cross-linked myosin heads showed pH-dependent Ca2+- and EDTA(K+)-ATPase activities similar to those of standard intact S-1. For measurements of myosin binding to actin, the modified myofibrils were digested with trypsin at a weight ratio of 1:50 under rigor, relaxed, and active-state conditions. Aliquots of tryptic digestion reactions were then cleaved with chymotrypsin to yield isolated myosin heads and their fragments. Analysis of the decay of myosin heavy-chain bands on SDS gels yielded the rates of myosin cleavage under all conditions and enabled the measurements of actomyosin binding in myofibrils in the presence of MgATP. Using this approach, we detected rigorlike binding of 25 +/- 6% of myosin heads to actin in myofibrils during ATP hydrolysis.     

A chemically synthesized radiolabeled signal peptide: design, preparation, and biological evaluation of an iodinated analog of preproparathyroid hormone

The chemically synthesized signal peptide (native-sequence signal peptide) of preproparathyroid hormone exhibits signal sequence-like activity by inhibiting the translocation/processing of precursor proteins to their mature forms in an in vitro translation system. In order to prepare a biologically functional radiolabeled form of this peptide, we undertook structure-function studies of the native-sequence signal peptide. Since conventional iodination of peptides is performed under oxidizing conditions, chemical design efforts were focused on the oxidation-labile residues, methionine and cysteine, present in the native sequence. Substitution of the three methionines with norleucine and the single cysteine with alanine yielded a surfur-free analog, [Nle-(-25), Nle-(-21),Nle-(-18),Ala-(-14),D-Tyr-(+1)]pre-proPTH-(-29-+1)amide, which is resistant to oxidation and active in the inhibition of processing assay. An interaction between the signal region and one of the components of the intracellular secretory apparatus, signal recognition particle (SRP), was demonstrated: iodinated sulfur-free analog was cross-linked (using the homo-bifunctional reagent disuccinimidyl suberate) to the 54 kilodalton (kDa) subunit of SRP. The 68 kDa and 72 kDa subunits of SRP were also labeled, but to a lesser extent, by the iodinated peptide.     

Demonstration of post-translational secretion of human placental lactogen by a mammalian in vitro translation system

This study demonstrates the post-translational translocation across the rough endoplasmic reticular membrane of a mammalian secretory protein, human preplacental lactogen. In the rabbit reticulocyte lysate, human preplacental lactogen biosynthesis is arrested by addition of cycloheximide prior to supplementation with dog pancreatic microsomal membranes, which have previously been shown to translocate and process nascent secretory proteins in a cotranslational manner. Twenty-five percent of the precursor protein is consistently converted to its mature form under these post-translational conditions. The resulting mature hormone is resistant to proteolytic degradation by added proteases, thus indicating that it is translocated across the microsomal membrane and sequestered within the lumenal space of the microsomal vesicles. Approximately one-half of the precursor protein synthesized is associated with the ribosomes. Only the ribosome-associated fraction is secreted in this in vitro system, suggesting that the process of post-translational secretion requires ribosomes for protein interaction with the elements of a subcellular secretory apparatus.     

The biological relevance of HPLC-purified vasoactive intestinal polypeptide monoiodinated at tyrosine 10 or tyrosine 22

Three major forms of monoiodinated VIP (M125I-VIP) were isolated after chloramine-T iodination and HPLC purification. The iodinated tyrosine residue was located in each form of M125I-VIP using arginase C and trypsin digestion for obtaining defined fragments containing only one tyrosine residue. The HPLC isolated iodinated fragments thus obtained were used for HPLC comigration studies with iodinated synthetic C and N terminal VIP fragments and for amino acid analysis. The first two eluting peaks 1 and 2 are (M125I-Tyr10-VIP); peak 1 has an oxidized methionine; peak 3 is a (M125I-Tyr22-VIP) which also has an oxidized methionine. A reduced counterpart of peak 3 named peak 4 was isolated by further HPLC analysis. The ability of the different species of M125I-VIP to stimulate adenosine cyclic 3',5'-phosphate (cAMP) production in transformed colonic cells in culture (HT-29) was compared to that of native VIP. The mean potencies of the M125I-VIP species expressed as a percentage relative to the potency of native VIP were, peak (1): 0.98; (2): 0.84; (3): 1.38; (4): 1.48, in the range of concentrations tested (2-60 pM). The M125I-Tyr22-VIP are significantly more active than native VIP (P less than 0.01). Oxidation of methionine or iodination of tyrosine 10 does not significantly modify the biological activity of VIP. We conclude that iodination of Tyr-22 located in the apolar helical COOH-terminal of VIP increases the effectiveness of VIP interaction with its receptors. Thus the tyrosyl residue and the localized hydrophobic features of VIP are critically involved in the function of this neurotransmitter.     

Cytochrome b561 catalyzes transmembrane electron transfer

Purified cytochrome b561 from bovine adrenal medulla chromaffin vesicles has been reconstituted into phosphatidylcholine vesicles by a detergent-dialysis method. When the reconstituted cytochrome-containing vesicles were preloaded with ascorbic acid and cytochrome c was added to the external medium, the internal ascorbic acid was able to reduce the external cytochrome c. This reduction of cytochrome c was dependent on the presence of cytochrome b561 in the membrane and was not due to leakage of ascorbate from the vesicles. These results demonstrate that cytochrome b561 catalyzes a transmembrane electron transfer.     

Intragranular vesicles: new organelles in the secretory granules of adrenal chromaffin cells

Summary Chromaffin granules from bovine adrenal medullary chromaffin cells have been found to contain small vesicular structures bounded by unit membranes. Detection of these intragranular vesicles within intact cells requires the use of quick-freezing methods. The intragranular vesicles are labile to fixation by aldehydes which explains why they have not been described in intact cells until now. They are found in approximately 60% of the dense-core chromaffin granules in cells and 85% of isolated granules. They are usually clustered in groups of one to as many as five between the core and the inner surface of the granule membrane. The intragranular vesicles are independent vesicles in that they do not appear as simple invaginations of the granule membrane in either serial thin-section or freeze-etch views. Furthermore, they are released from the cell along with granule contents during nicotine-induced secretion of catecholamines. The structural heterogeneity provided by the intragranular vesicles may be related to the functional heterogeneity of granule contents observed in many recent biochemical studies.