Low molecular weight pool

Ten days after injection of label, the low-molecular-weight pool contained virtually no label, whereas label remained associated with phosphorylase. Subsequent isolation of phosphorylase from different animals over a range of time periods between 10 and 30 days defined an exponential decay, the rate constant of which was taken as the rate of degradation of phosphorylase. This was subsequently confirmed by independent measurement of the rate of turnover of the enzyme using continuous infusion of labeled amino acids—a method that is independent of reutilization artifacts. The rate of turnover of phosphorylase was the same when measured by either method (Beynon et al., 1986 Cookson and Beynon, 1989). 

NADH, which enters the Krebs cycle. However, during cerebral ischaemia, metabolism becomes anaerobic, which results in a precipitous decrease in tissue pH to below 6.2 (Smith etal., 1986 Vonhanweh etal., 1986). Tissue acidosis can now promote iron-catalysed free-radical reactions via the decompartmentalization of protein-bound iron (Rehncrona etal., 1989). Superoxide anion radical also has the ability to increase the low molecular weight iron pool by releasing iron from ferritin reductively (Thomas etal., 1985). Low molecular weight iron species have been detected in the brain in response to cardiac arrest. The increase in iron coincided with an increase in malondialdehyde (MDA) and conjugated dienes during the recirculation period (Krause et al., 1985 Nayini et al., 1985). 

Stimulation of murine peritoneal macrophages with IFNy and LPS induced NO synthesis and activated IRE binding by IRP-1 and IRP-2. This activation is NO dependent and accompanied by a loss of the aconitase activity of IRP-1. This was also shown to occur in other cell types, such as the erythroid cell line K562, rat brain slices and mouse fibroblast lines and did not require cytokine stimulation. The activating effects of NO may depend on a direct interaction with the 4Fe-4S cluster or a slow effect on the low-molecular-weight iron pool. Activation of IRP-2 by LPS and IFN-y has not been universally confirmed (reviewed by Cairo and Pietrangelo, 2000). 

To aid in the characterization of the DOM pool, marine organic chemists have developed techniques fiar separating compounds into size fractions. Tangential flow ultrafiltration is used to isolate a high-molecular-weight (HMW) fraction from a low-molecular-weight (LMW) fraction. The size cutoff between these is approximately 1 tun, which equates... 

DOM is also released into seawater by phytoplankton fiar reasons that are as yet imclear. On average, 13% of the phytoplankton carbon is released as exudates, some of which are low-molecular-weight compounds, such as free amino acids and tricarboxylic acids. Other exudates are high-molecular-weight compoimds, such as the acylated heteropolysaccharides. These macromolecules are relatively chemically resistant and appear to form a large portion of the HMW DOC pool. 

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