A heterotrophic desert stream? The role of sediment stability

In autumn 1998 stream metabolism was measured in the Hassayampa River, Arizona, a Sonoran Desert stream, using single-station diel oxygen curves and an oxygen mass balance model. Oxygen consumption rates of parafluvial and channel sediments were determined with respiration chambers. Bedload of channel sediments (sand) prevented significant primary production by benthic autotrophs, despite favorable nutrient, light, and temperature conditions. Ecosystem respiration was relatively low (1.50 g O 2 m -2 d -1 ) and presumably fueled by production in the riparian zone and riverine marshes. Respiration rates in the parafluvial zone and in channel sediments ranged from 0.6 to 1.4 g O 2 m -3 sediment h -1 . Sediment organic matter (ash-free dry mass) was 4.0 ± 1.8 kg m -3 sediment and did not significantly differ between the channel and the parafluvial zone. Results indicate that heterotrophic processes may dominate the metabolism of desert stream segments over extended periods of time if unstable sandy bed sediments prevail.     

Activation of the carcinogen N-hydroxy-2-acetylaminofluorene by rat mammary peroxidase

A peroxidase preparation from rat mammary gland parenchymal cells, a target tissue of arylamine carcinogens, was shown to activate the carcinogen N-hydroxy-2-acetylaminofluorene via a nitroxyl free radical intermediate to the more active carcinogens nitrosofluorene and N-acetoxy-2-acetylaminofluorene. Hydrogen peroxide, cumene hydroperoxide, and linoleic acid hydroperoxide were effective as substrates. The antioxidants ascorbate, propyl gallate and reduced glutathione prevented the free radical activation route.     

Structural changes in glycogen phosphorylase induced by phosphorylation

A comparison of the refined crystal structures of dimeric glycogen phosphorylase b and a reveals structural changes that represent the first step in the activation of the enzyme. On phosphorylation of serine-14, the N-terminus of each subunit assumes an ordered helical conformation and binds to the surface of the dimer. The consequent structural changes at the N- and C-terminal regions lead to strengthened interactions between subunits and alter the binding sites for allosteric effectors and substrates.     

脑肿瘤细胞的分子标记

在过去的20年，科研工作者已经在分子肿瘤领域取得了划时代的重大发现，这很大程度上归功于免疫组织化学方法的应用。这一方法在脑肿瘤研究中也已经从试验阶段走向了成熟。