Complement activation by parasites. A review

Activation of complement by parasites (living parasites or purified parasite antigens) is involved in several mechanisms of the host parasite relationship. In most of the experiments performed in vitro, complement activation was found to be lethal for the parasites, but sometimes it could be essential for the development of parasitemia. Both classical and alternative complement pathways may be activated by parasites; the classical pathway nearly always requires the involvement of antibodies whereas the alternative pathway is activated directly by products released by the parasites or present in their teguments. Activation of complement, especially via the alternative pathway may also be a prerequisite for cellular adherence to parasites which can then cause their death.

Abnormalities of triiodothyronine binding to lymphocyte and fibroblast nuclei from a patient with peripheral tissue resistance to thyroid hormone action

T3 binding to lymphocyte nuclei has been studied in normal individuals and in a patient (MaG) with peripheral resistance to thyroid hormone action. This syndrome is defined by the presence of hypothyroidism or euthyroidism with high plasma levels of thyroid hormone. T3 bound to a single set of binding sites in normal adult lymphocyte nuclei with a mean Ka of 8.9 +/- 7.1 x 109 M-1, and a capacity of 4.4 +/- 2.9 fmol/100 micrograms DNA. A single binding site was also disclosed in MaG's lymphocytes with a Ka of 0.43 x 109 M-1 and a capacity of 10.5 fmol/100 micrograms DNA. This low affinity was not due to the presence of high plasma T3 level in the patient, since administration of 100 micrograms T3 to normal adult volunteers induced the presence of two different binding sites. The mechanism responsible for this phenomenon is unknown. To binding was also studied using cultured fibroblasts which were incubated in serum-less medium before the binding experiments. One single binding site (Ka, 1.9 x 10(10) M-1, capacity, 12.9 fmol/100 micrograms DNA) was detected in normal fibroblast nuclei. In contrast, a curvilinear Scatchard plot was obtained when MaG's fibroblasts were used. This result could be compatible with the presence of either two different binding sites or negative cooperativity. In support of the latter possibility, Hill plots gave a number lower than unity. The results suggest that the syndrome of peripheral tissue resistance to thyroid hormone action due to a defect at the level of the nuclear receptor. The possible existence of similar syndromes due to an alteration at the level of a post-T3-binding mechanism is not eliminated.

Thyroid hormone receptors from liver nuclei: characteristics of receptor from normal, thyroidectomized, and triiodothyronine-treated rats; measurement of occupied and unoccupied receptors, and chromatin binding of receptors

The capacity and characteristics of liver nuclear receptors for T3 have been compared in normal rats, thyroidectomized rats, and thyroidectomized rats treated with different doses of T3. Receptor capacity was unchanged, with the exception of a decrease of total nuclear receptors in rats treated with 15 microgram T3/day, which is believed due to lack of complete dissociation of the endogenously bound hormone during incubation in vitro. During incubation of the isolated nuclei in 0.32 M sucrose plus 1 mM MgCl2 plus 20 mM Tris-Cl buffer, pH 7.85, about 50% of the nuclear receptors were released to the medium and no effect of T3 treatment on the amount released was found. The affinity of the receptors free in solution was 50% higher than that of receptors retained in the nuclei. Receptors from normal and thyroidectomized rats were studied by ultracentrifugation through sucrose gradients. No difference in sedimentation coefficient was found between the groups of rats, whether the receptors were filled with the labeled hormone before or after centrifugation and separation of the gradient fractions. Occupied and unoccupied receptor capacity was estimated by incubation of receptor preparations at 20 and 0 C for 2 h. Observed capacity at 20 C represents unoccupied plus approximately 80% of occupied sites, while observed capacity at 0 C represents unoccupied plus 10% of occupied site. Binding of receptors to the chromatin was studied by incubating nuclear extract from normal rats with chromatin and measuring the amount of receptor that was not bound and remained free in the supernatant. Receptor not bound to chromatin was incubated at 0 and 20 C, with a saturating concentration of [125I]T3, to measure occupied and unoccupied sites. Both occupied and unoccupied receptor decrease in the supernatant after incubation with the chromatin when the concentration of chromatin in the incubation medium is increased. No saturation of the chromatin-binding sites was achieved by increasing the concentration of receptor several-fold, and no difference in the binding of occupied and unoccupied receptors to chromatin was observed. These data rule out the possibility that the concentration of receptors in the nucleus is under control of the hormone. Major molecular changes of the receptor that could be induced by T3, such as dissociation into subunits, are also ruled out. Binding studies to the chromatin are not necessarily conclusive, as nonspecific binding may be masking the physiological binding of the receptors to a limited number of acceptor sites.

Conformational calculations of some azobilirubin isomers

A theoretical conformational analysis of one benzeneazodipyrromethene isomer of bilirubin in both the E and Z forms was performed by the PCILO method. The Z arrangement in its planar conformation showed the highest stability for this type of configurations.

Thyroid hormone receptors: release of receptor to the medium during in vitro incubation of isolated rat liver nuclei

Isolated rat liver nuclei show a substantial amount of T3 receptor release to the medium during in vitro incubation. This has been shown to be a general feature of nuclei compared after several methods of isolation and incubation. About 50% of nuclear receptors are released to the medium when incubated in sucrose-MgCl2-Tris, pH 7.85, for 2 h at 20 C.DNA, histones, and non-histone proteins (NHP) are also released. CaCl2 inhibits about 90% of the release of DNA and histones, but has less effect on inhibiting leakage of NHP and nuclear T3-binding protein (NTBP). The highest leakage for each fraction was found when incubating nuclei in the presence of EDTA. The receptor released to the medium has an affinity virtually identical to the receptor remaining in the nuclei. At least 1 mM dithiothreitol is needed to avoid degradation of the receptor. The NTBP has a sedimentation constant of 4.5 S when studied in low ionic strength gradients. Increasing KCl concentration decreases progressively its sedimentation constant, and in gradients containing 0.4 M KCl the receptor sediments as a single peak of 3.4 S. Since release of receptor to incubation medium decreases free T3 concentration, it must be taken into account in calculating receptor affinity. Total nuclear capacity in vitro is obviously underestimated, unless receptor released to medium is measured. Receptor exchange between cytosol and nucleus may be of physiologic significance.

The action of thyroid hormone

Thyroid hormone affects both developmental and metabolic processes. It has a relatively specific effect on the synthesis of a number of enzymes and other proteins. The fundamental cellular mechanism of action seems to be at the level of genetic regulation. It involves interaction with nuclear receptors, leading to an activation of the protein synthesizing machinery. How binding to receptors is coupled to genetic activation is completely unknown. At least part of the metabolic effects of thyroid hormone could be mediated through an interaction with mitochondria and cell membrane, and with some enzymatic systems such as adenylcyclase.

Effect of the exposure to cold on the extrathyroidal conversion of L-thyroxine to triiodo-L-thyronine, and on intramitochondrial alpha-glycerophosphate dehydrogenase activity in thyroidectomized rats on L-thyroxine

Thyroidectomized rats have been injected daily with 125-I labelled L-thyroxine (T4) and, once isotopic equilibrium was attained, divided into cold-exposed (4-10 degrees C) and control (21-24 degrees C) groups, the daily T4 administration being continued till the end of the experiment. Fourteen days after onset of cold exposure, the total I of different organs and of the carcass was determined and the tissues submitted to extraction and paper chromatography for the separation of T4 and T4-derived I-containing compounds. The Activity of intramitochondrial alpha-glycerophosphate dehydrogenase (alpha-GPD) was measured in kidneys and liver. It was found that the total amount of I was intensely decreased in all samples from cold-exposed animals. The proportion of this Iwhich was non-extractable was the same, in all tissues, for cold-exposed and control rats. The % of extractable tissue radioactivity in the form of T4 was decreased, and that found as T4-derived T3 was increased, in all samples from cold-exposed animals. The T3/T4 ratio was increased more than two-fold in all tissues studied. The concentration of T4 decreased significantly in all tissues, whereas the concentration of T3 in tissues of cold-exposed rats did not decrease. It actually increased in kidneys and lungs, and remained the same in liver and carcass. Despite the decrease in the concentration of T4 in the kidneys, alpha-GPD activity was increased in this tissue, where the concentration of T3 was increased. No change in the alpha-GPD activity was found for the liver, where the concentration of T3 was the same for cold-exposed and control rats. Thus, it appears likely that the conversion of T4 AND T3 is increased by the exposure to cold of thyroidectomized rats on a constant dose of T4.alpha-GPD activity in a given tissue appears to be more closely related to the concentration of T3, than to that of T4.

Inhibition by propylthiouracil of the extrathyroidal formation of triiodothyronine from thyroxine

Thyroidectomized rats were injected daily with 1.9 μg/100 g body weight of 125I-labelled L-thyroxine (T*4). When isotopic equilibrium was attained, half the experimental group was injected twice with 2 mg of propylthiouracil at a 24 h interval. Animals were killed 7 h after the last dose and the amounts of T*4 and labelled iodide and T3 in peripheral tissues were determined after extraction and paper chromatography. The amount of T3 derived from T4 is significantly lower in tissues of rats receiving PTU.