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Melanin biosynthesis (melanogenesis) is a metabolic pathway exclusively expressed by melanocytes and melanoma cells, and is often altered and/or markedly elevated in the latter cells. The changes in melanogenesis may be responsible for some of the clinical and histopathological features unique to melanoma. Melanogenesis may also contribute to the malignant transformation of melanoma precursors (i.e., atypical moles [or dysplastic nevi]) to melanoma as seen in patients with the familial atypical multiple-mole-melanoma (FAMMM) syndrome. However, it does not appear to affect the multi-step growth phases of melanoma cells from radial to vertical and lastly metastatic growth phases. Within the melanosomal compartment, eu- and pheomelanin pigments are synthesized. Both tyrosinase and lysosome-associated membrane protein (LAMP) gene products play important roles in this process. A coordinated interaction between these two gene family products is required for melanogenesis to occur properly. p90 calnexin is a new melanosome-associated molecule that is presumed to function as a melanogenesis chaperone by controlling the assembly and folding of glycoprotein intermediates of tyrosinase and LAMP gene families. 相似文献
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D. J. Blundell R. A. Crick B. Fife J. Peacock A. Keller A. Waddon 《Journal of Materials Science》1989,24(6):2057-2064
PEEK/carbon fibre composites (derived from APC-2) have been examined with a permanganic etching technique in order to reveal
the crystalline, spherulitic morphology of the thermoplastic PEEK polymer matrix. The locations of the nucleation sites for
spherulite growth have been categorized. Nucleation can occur both within the matrix and from the carbon fibres. Crystallization
at lower temperatures favours matrix nucleation. Nucleation from fibres is dominated by sites associated with fibre-fibre
contact. There is no evidence of “transcrystalline” growth. The study also identifies two types of crystal orientation effect
in the polymer matrix. The first is a slight orientation that can occur in standard mouldings and is the result of the fibres
constraining the shape of the volume into which spherulite growth can occur. The second effect produces abnormally high crystal
orientation and is the result of improper processing at too low a melt temperature. Such conditions cause self-seeding during
consolidation of laminates which, coupled with flow-induced orientation, can lead to directionally arranged spherulite precursors
in different stages of morphological development. 相似文献
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MV Nakariakova TA Antonenko TG Khomutova MG Massova TK Konstantinova EG Gershte?n 《Canadian Metallurgical Quarterly》1996,(3):15-17
Inhaled Orlok causes changes in the CNS, blood morphology, renal function. The threshold concentration of Orlok is 0.17 mg/m3, its subthreshold concentration is 0.023 mg/m3, MAC in the workplace air is 0.05 mg/m3. 相似文献
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Klebsiella aerogenes UreE, one of four accessory proteins involved in urease metallocenter assembly, contains a histidine-rich C terminus (10 of the last 15 residues) that is likely to participate in metal ion coordination by this nickel-binding protein. To study the function of the histidine-rich region in urease activation, ureE in the urease gene cluster was mutated to result in synthesis of a truncated peptide, H144* UreE, lacking the final 15 residues. Urease activity in cells containing H144* UreE approached the activities for cells possessing the wild-type protein at nickel ion concentrations ranging from 0 to 1 mM in both nutrient-rich and minimal media. In contrast, clear reductions in urease activities were observed when two ureE deletion mutant strains were examined, especially at lower nickel ion concentrations. Surprisingly, the H144* UreE, like the wild-type protein, was readily purified with a nickel-nitrilotriacetic acid resin. Denaturing polyacrylamide gel electrophoretic analysis and N-terminal sequencing confirmed that the protein was a truncated UreE. Size exclusion chromatography indicated that the H144* UreE peptide associated into a homodimer, as known for the wild-type protein. The truncated protein was shown to cooperatively bind 1.9 +/- 0.2 Ni(II) ions as assessed by equilibrium dialysis measurements, compared with the 6.05 +/- 0.25 Ni ions per dimer reported previously for the native protein. These results demonstrate that the histidine-rich motif is not essential to UreE function and is not solely responsible for UreE nickel-binding ability. Rather, we propose that internal nickel binding sites of UreE participate in urease metallocenter assembly. 相似文献