Inhibition of mitochondrial respiratory function by an organic solvent extractable component from an extract of burn eschar

Role of endogenous TRPV1 agonists in a postburn pain model of partial-thickness injury

Oxidized linoleic acid metabolites (OLAMs) are a class of endogenous transient receptor potential vanilloid 1 (TRPV1) channel agonists released on exposure of tissue to transient noxious temperatures. These lipid compounds also contribute to inflammatory and heat allodynia. Because persistent pain after a burn injury represents a significant clinical challenge for treatment, we developed an in vivo rat model of partial-thickness cutaneous thermal injury and examined whether TRPV1 and specific OLAM metabolites play a role in mediating postburn pain injury. This peripheral model of burn injury had marked thermal allodynia peaking at 24h after thermal injury, with allodynia being maintained for up to 7d. Immunohistochemical characterization of tissue taken from injury sites revealed an increase in leukocyte/macrophage infiltration that was colocalized with TRPV1-positive fibers. Using this peripheral thermal injury model, we found that pharmacological blockade of peripheral TRPV1 receptors reduced thermal allodynia by about 98%. Moreover, there was a significant increase in OLAM levels compared to naive controls in hind paw skin biopsies. Additional studies of the metabolism of [C(14)]-linoleic acid in skin biopsies revealed the role of the cytochrome P450 (CYP) system in mediating the metabolism of linoleic acid after thermal injury. Finally, we demonstrated that direct inhibition of OLAMs using OLAM antibodies and indirect inhibition using the CYP inhibitor ketoconazole significantly reduced postburn thermal allodynia. Collectively, these findings point to a novel role of the OLAMs and CYP-related enzymes in generating postburn allodynia via activation of peripheral TRPV1.

Postburn trauma insulin resistance and fat metabolism

Hyperglycemia and insulin resistance have long been recognized in severe burn patients. More recently, it has been observed that controlling hyperglycemia, or alleviating insulin resistance, is associated with improved outcomes. This has led to a renewed interest in the etiology of insulin resistance in this population. The postinjury hyperglycemic response appears to be associated with multiple metabolic abnormalities, such as elevated basal energy expenditure, increased protein catabolism, and, notably, significant alterations in fat metabolism. The synergy of all of the responses is not understood, although many studies have been conducted. In this article we will review the present understanding of the relationship between fat metabolism and insulin resistance posttrauma, and discuss some of the recent discoveries and potential therapeutic measures. We propose that the insulin resistance is likely related to the development of "ectopic" fat stores, i.e., triglyceride (TG) storage in sites such as the liver and muscle cells. Deposition of TG in ectopic sites is due to an increase in free fatty acid delivery secondary to catecholamine-induced lipolysis, in conjunction with decreased beta-oxidation within muscle and decreased hepatic secretion of fats. The resultant increases in intracellular TG or related lipid products may in turn contribute to alterations in insulin signaling.

Temporal changes in cellular energy following burn injury

Availability of ADP is a predominant influence on respiratory control. Associated with severe burn injury is an increase in energy expenditure. The purpose of this study was to determine the temporal changes in ATP, ADP, NAD, and NADH following severe burn and thereby assess any related alterations in respiratory control and energy deficit. During isoflurane anesthesia and following intraperitoneal injection of saline, 32 mice were flame burned at 40% body surface area. Twelve mice served as controls. At 12, 24, 72, and 168 h post-burn, groups of mice underwent celiotomy with determination of hepatic surface blood flow using laser Doppler and oxygen saturation using pulse oximetry. Biopsies of liver were then frozen in liquid nitrogen for subsequent quantification of ATP, ADP, AMP, NAD, and NADH by HPLC. Mortality was 12.5% at 72 h post-burn and 25% at 1 week. Oxygen saturation and hepatic surface blood flow remained similar to control values throughout the week after burn. ATP, ADP, and energy charge decreased progressively following burn reaching a significant decrease from unburned controls at 72 h. Availability of NADH remained statistically similar to unburned controls throughout the week after burn. These results demonstrate that despite maintenance of baseline oxygen delivery, there was a nadir in ATP and ADP availability and energy charge in the liver at 72 h after burn. This finding supports the concept of a limitation in phosphorylation after injury. Availability of NADH remained at or above pre-burn concentrations suggesting that the rate of fuel oxidation was not a limiting factor for ongoing oxidative phosphorylation for energy.

Lipid peroxidation of mitochondrial membrane induced by D1: an organic solvent extractable component isolated from a crude extract of burn eschar

D1, an organic solvent extractable component of an extract from burn eschar was characterized and the mechanism of its inhibitory effect on mitochondrial respiratory functions was investigated. Using silicic acid column chromatography, it was found that D1 consisted of two parts: a toxic simple lipid and a non-toxic complex lipid. Six subunits were obtained by further silicic acid column chromatography, among them, the no. 4 and no. 6 were toxic. Infrared spectrometric studies showed that no. 4 consisted of esters, while no. 6 were peroxides. D1 also contained large amounts of malonaldehyde (MDA) and lipid hydroperoxides (ROOH). Vitamin E was found to prevent the inhibitory action of D1 on mitochondrial function and to reduce the amount of MDA produced. However, if vitamin E was introduced after the addition of D1 the inhibition could not be prevented, although there was still a reduction in MDA production; therefore, MDA per se was probably not the cause of the inhibition. D1 induced ROOH formation in mitochondrial membranes. Cumene hydroperoxide, an organic hydroperoxide, was capable of inhibiting the mitochondrial function. This inhibitor action was blocked by vitamin E. It is speculated that ROOH in D1 is probably the element that inhibits mitochondrial function. That D1 caused lipid peroxidation of membrane lipid was also proved by analysing the fatty acid composition of mitochondrial membranes before and after treatment with D1. It was found that the amount of polyunsaturated fatty acids decreased and that of saturated fatty acids increased after incubation with D1, a direct proof that lipid peroxidation has occurred. Lipid peroxidation of the membrane lipid was the result of the action of oxygen free radicals. This process was best shown by a chemiluminescence study. In this experiment, D1 induced chemiluminescence which was dose-dependently related to the amount of D1 used. This was probably the most direct proof that D1 caused lipid peroxidation of membrane lipid resulting in damage of the mitochondria.

Sites of inhibition of mitochondrial electron transport by D1, an organic solvent extractable component from burn eschar

The inhibitory sites of component D1, isolated by extracting burn eschar with ethyl acetate, on rat liver mitochondrial respiratory chain was investigated. By using changes of the redox state of NADPH in various respiratory states, the effect of D1 on electron flow through the phosphorylation site 1 was studied. It was found that D1 did not interfere with electron transport through site 1, nor did it inhibit the activity of NADH:duroquinone reductase. The functional state of site 2 was tested directly by using duroquinol as an artificial electron donor and cytochrome c as electron acceptor. D1 inhibited the activity of this enzyme in a dose-dependent manner. The redox state of the electron carrier cytochrome b was not influenced by D1. However, a transient process of oxidation of cytochrome c1 was demonstrated, suggesting a brief inhibitory process in the reducing side of it. Two inhibitory sites in site 2 were found by studying the TMPD and DCIP bypasses. One was at the reducing side of cytochrome c1, and the other was at the reducing side of cytochrome b. Further studies are needed to find out the exact position and nature of these inhibitory sites.