Contents Protein Carriers

In support of this contention, the carrier protein Aphrodisin makes an early appearance in vaginal secretions. In pre-pubertal hamsters, it thus indicates chemosensory preparation for the onset of female maturity (Magert, 1999). The proven ability of the AOS to modulate the CNS-pituitary-gonadal axis by advancing or retarding endocrine activity (Chap. 5), underlines its role as primarily the chemosensor of the reproductive system. The adaptive consequence of responses, which allows an avoidance of premature breeding, or of a postponement of puberty, would seem to be advantageous. 

To treat the malabsorption and subsequent diarrhea, lipid-poor diets (<5 g/day) should be implemented with a restriction of triglycerides containing long-chain fatty acids. Medium-chain fatty acids rely on other protein carriers besides apoB (i.e., albumin) for plasma transport, making them an ideal lipid substitute. However, long-term supplementation should be cautioned as associated hepatic fibrosis could occur. Diets should also contain increased protein and carbohydrate content to compensate for caloric loss from fat restriction. 

Polyanhydrides have also been investigated as protein carriers.f Poly(SA) and 20 80 CPH SA copolymer microspheres were foimd to conserve both the primary structure of the released protein [bovine serum albumin (BSA)] and the secondary structure of the encapsulated and released protein, and showed a sustained delivery for approximately 15 and 30 days, respectively. As the CPH content in the copolymer increased, the secondary structure of BSA was not conserved, as indicated by the steep decrease in the a-helix content. 

Jervis used porous silica coated with chemisorbed polyacrylhydrazide for immobilization of adenosine monophosphate (AMP) [117]. After periodate oxidation of its ribose residue the ligand was coupled to the carrier and used for isolation of lactate dehydrogenase from rabbit muscle. The specific capacity was 2 mg of protein/g adsorbent with a ligand content of 10 pmol/g, whereas recovery of enzymatic activity after elution was 85%. Hipwell et al. [118] found that for effective binding of lactate dehydrogenases on AMP-o-aminoalkyl-Sepharose the spacer arm length required at least 4 methylene links. Apparently, a macromolecule of polyacrylhydrazide acts itself like an extended spacer arm and thus allow AMP to bind the enzyme. 

Although anosmias to these compounds occur at similar levels, some communicative value may arise from the persistence of signal emissions which are not enantiomerically pure (Carman, 1993 Wysocki et al., 1999). In secreted mixtures, the alternate versions of such compounds are produced in a constant ratio since they have identical volatility and hence provide stable informational content to the receiver. Support for this idea comes from the results of the NMR mapping of the BT binding site within the MUP1 carrier (Zidek et al., 1999). Here the protein-ligand complex does exist in the expected ratio, and for both enantiomers, although the orientation of the bound thiazole was interpreted as opposite to that indicated by previous X-ray analyses. 

The amino acid L-tryptophan is the precursor for the synthesis of 5-HT. The synthesis and primary metabolic pathways of 5-HT are shown in Figure 13-5. The initial step in the synthesis of serotonin is the facilitated transport of the amino acid L-tryptophan from blood into brain. The primary source of tryptophan is dietary protein. Other neutral amino acids, such as phenylalanine, leucine and methionine, are transported by the same carrier into the brain. Therefore, the entry of tryptophan into brain is not only related to its concentration in blood but is also a function of its concentration in relation to the concentrations of other neutral amino acids. Consequently, lowering the dietary intake of tryptophan while raising the intake of the amino acids with which it competes for transport into brain lowers the content of 5-HT in brain and changes certain behaviors associated with 5-HT function. This strategy for lowering the brain content of 5-HT has been used clinically to evaluate the importance of brain 5-HT in the mechanism of action of psychotherapeutic drugs. 

Carriers can also, in theory, be isolated from the membrane and quantified using gel electrophoresis (or similar techniques), although in practice, it is only possible to measure a protein content and not a carrier number (e.g. [196]). Note that in all of the cases mentioned above, carrier numbers are generally quantified by assuming a 1 1 stoichiometry with a labelled solute. 

The inhibition of amino-acid transport has been regarded as the main toxic effect of mercury compounds [82], The biochemical mechanism underlying the inhibition is unclear. In unfertilized sea-urchin eggs an interaction with the amino-acid carrier was found, whereas in fertilized eggs inhibition of amino-acid transport was indirect and might result from an elevation of the Na + content of the egg caused by the inhibition of the Na+ pump [83]. The action on the diffusional process could be mediated by an effect on membrane phospholipids or on membrane proteins, or by interaction with Ca2+ which stabilizes membrane structure. Mercuric chloride in skate liver cells inhibited amino acid transport, decreased Na + /K + -ATPase (adenosinetriphosphatase) activity, impaired volume regulatory mechanisms and increased the permeability of the plasma membrane to potassium [84]. It has been suggested that... 

Iron-sulfur proteins are non-heme electron carriers present in a wide range of living organisms and are known to cover different and important roles in biological processes. They will be treated here in order of their increasing iron content. 

The information content of such genes suffices only for relatively small proteins, certainly not optimally adapted. This means, however, that these are by now long outdated as information carriers and have been... 

Hirai A, Kino T, Tokinaga K, Tahara K, Tamura Y, Yoshida S. Regulation of sterol carrier protein 2 (SCP2) gene expression in rat peritoneal macrophages during foam cell formation. A key role for free cholesterol content. J Clin Invest 1994 94 2215-2223. 

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