Rates of dissociation of steroid and thyroid hormones from human serum albumin

Low Urinary Free Cortisol as a Risk Factor for Patients with Variceal Bleeding

Background and Objectives: Specificity and reliability issues of the current cortisol assessment methods lead to limitations on the accurate assessment of relative adrenal insufficiency. Although free cortisol provides a more accurate evaluation of adrenal cortisol production, the expense and time-consuming nature of these assays make them impractical for routine use. Research has, thus, focused on alternative methods, such as indirectly measuring free cortisol using Coolens' equation or directly assessing salivary cortisol concentration, which is considered a more favorable approach despite associated challenges like sampling issues and infection risks. The aim of this study was to explore correlations between 24 h urinary free cortisol (UFC), free plasma cortisol, serum total cortisol, and salivary cortisol as potential reliable indices of free cortisol in the setting of variceal bleeding. Additionally, we assessed the predictive value of UFC for 6-week mortality and 5-day treatment failure in patients with liver cirrhosis and variceal bleeding. Materials and Methods: A total of 40 outpatients with liver cirrhosis and variceal bleeding were enrolled. Free cortisol levels in serum, saliva, and urine were assessed using the electrochemiluminescence immunoassay method. For the measurement of plasma-free cortisol, a single quadrupole mass spectrometer was employed. The quantification of free cortisol was fulfilled by analyzing the signal response in the negative ESI-MS mode. Results: UFC was significantly correlated to free plasma cortisol. Negative correlations were demonstrated between UFC, the Child-Pugh (CP) score, and C reactive protein (CRP) levels. In the multivariate analysis, CP stage C was associated with 6-week mortality risk and portal vein thrombosis with 5-day treatment failure using Cox regression and binary logistic regression analyses, respectively. Patients who experienced rebleeding, infection, or death (or any combination of these events) presented with lower levels of UFC. Conclusions: This study suggests that low levels of UFC may impose a risk factor for patients with liver cirrhosis and variceal bleeding. The use of UFC as an index of adrenal cortisol production in variceal bleeding warrants further investigation.

Intrathyroidal feedforward and feedback network regulating thyroid hormone synthesis and secretion

Thyroid hormones (THs), including T4 and T3, are produced and released by the thyroid gland under the stimulation of thyroid-stimulating hormone (TSH). The homeostasis of THs is regulated via the coordination of the hypothalamic-pituitary-thyroid axis, plasma binding proteins, and local metabolism in tissues. TH synthesis and secretion in the thyrocytes-containing thyroid follicles are exquisitely regulated by an elaborate molecular network comprising enzymes, transporters, signal transduction machineries, and transcription factors. In this article, we synthesized the relevant literature, organized and dissected the complex intrathyroidal regulatory network into structures amenable to functional interpretation and systems-level modeling. Multiple intertwined feedforward and feedback motifs were identified and described, centering around the transcriptional and posttranslational regulations involved in TH synthesis and secretion, including those underpinning the Wolff-Chaikoff and Plummer effects and thyroglobulin-mediated feedback regulation. A more thorough characterization of the intrathyroidal network from a systems biology perspective, including its topology, constituent network motifs, and nonlinear quantitative properties, can help us to better understand and predict the thyroidal dynamics in response to physiological signals, therapeutic interventions, and environmental disruptions.

Urinary free cortisol is a reliable index of adrenal cortisol production in patients with liver cirrhosis

BACKGROUND

The measurement of total and free cortisol has been studied as a clinical index of adrenal cortisol production in patients with liver cirrhosis. Correlations between free plasma and salivary cortisol have previously been reported in stable cirrhotic patients. Urinary free cortisol constitutes an index of adrenal cortisol production; however, it has never been used in assessing adrenal function in patients with liver cirrhosis.

AIMS

The aim of this observational study was to determine associations between urinary free cortisol, serum total, salivary, measured and calculated plasma free cortisol levels in cirrhotics, determining which of them can be used as an indirect index of free cortisol levels. Moreover, we investigated the potential use of 24 h urinary free cortisol as a prognostic factor for mortality.

METHODS

Seventy-eight outpatients with liver cirrhosis were included. Serum, salivary and urinary free cortisol were measured using the electrochemiluminenscence immunoassay. Plasma free cortisol determination was conducted using a single quadrupole mass spectrometer. The quantification of free cortisol was achieved by determining the signal response on negative ESI-MS mode.

RESULTS

Twenty-four hour urinary free cortisol levels correlated with free cortisol determined by mass spectrometer, total cortisol and calculated free cortisol levels. Patients with low levels of urinary free cortisol presented a significantly higher mortality rate compared to those with high levels. The factors associated with death risk were determined by Cox regression. In the multivariate analysis, two models were applied; in the first model, CP score, PVT and urinary free cortisol were found to be significantly related to patients' survival, whereas in the second, MELD score, ascites and urinary free cortisol were independently related to survival.

CONCLUSIONS

This study suggests that 24 h urinary free cortisol could be considered as a potential index of adrenal cortisol production in patients with liver cirrhosis and it potentially detects patients with a high mortality risk.

Treatment with Intravenous Methylprednisolone in Patients with Graves' Orbitopathy Significantly Affects Adrenal Function: Assessment of Serum, Salivary Cortisol and Serum Dehydroepiandrosterone Sulfate

Treatment of active, moderate-to-severe Graves’ orbitopathy (GO) is the administration of intravenous methylprednisolone (IVMP). IVMP may be followed by additional therapy with oral prednisone. The aim of this study was to analyze the impact of IVMP on adrenal function by evaluation of serum, salivary cortisol and serum dehydroepiandrosterone sulfate (DHEA-S). Fourteen patients received IVMP treatment (cumulative dose of 4.5 g in 12 weekly infusions) followed by oral prednisone (for three months). All patients showed normal adrenal function before the 12th IVMP pulse and one patient was diagnosed with secondary adrenal insufficiency (AI) after prednisone treatment. DHEA-S was significantly lower before the 12th IVMP pulse and after oral prednisone (p = 0.015 and p = 0.00002, respectively) in comparison to evaluation before therapy. DHEA-S levels were below the reference range in one and three patients before the 12th IVMP pulse and after prednisone therapy, respectively. We observed decreased serum (p = 0.05) and salivary (p = 0.011) cortisol levels after oral prednisone therapy in comparison to evaluation before therapy. Treatment with IVMP in a cumulative dose of 4.5 g affects adrenal function, causing more severe impairment of DHEA-S secretion than that of cortisol but does not cause secondary AI. Additional therapy with oral glucocorticoids after IVMP can cause secondary AI.

Spectroscopic evaluation of synthesized 5β-dihydrocortisol and 5β-dihydrocortisol acetate binding mechanism with human serum albumin and their role in anticancer activity

Our study focus on the biological importance of synthesized 5β-dihydrocortisol (Dhc) and 5β-dihydrocortisol acetate (DhcA) molecules, the cytotoxic study was performed on breast cancer cell line (MCF-7) normal human embryonic kidney cell line (HEK293), the IC₅₀ values for MCF-7 cells were 28 and 25 μM, respectively, whereas no toxicity in terms of cell viability was observed with HEK293 cell line. Further experiment proved that Dhc and DhcA induced 35.6 and 37.7% early apoptotic cells and 2.5, 2.9% late apoptotic cells, respectively, morphological observation of cell death through TUNEL assay revealed that Dhc and DhcA induced apoptosis in MCF-7 cells. The complexes of HSA-Dhc and HSA-DhcA were observed as static quenching, and the binding constants (K) was 4.7 ± .03 × 10⁴ M^-1 and 3.9 ± .05 × 10⁴ M^-1_, and their binding free energies were found to be -6.4 and -6.16 kcal/mol, respectively. The displacement studies confirmed that lidocaine 1.4 ± .05 × 10⁴ M^-1 replaced Dhc, and phenylbutazone 1.5 ± .05 × 10⁴ M^-1 replaced by DhcA, which explains domain I and domain II are the binding sites for Dhc and DhcA. Further, FT-IR, synchronous spectroscopy, and CD results revealed that the secondary structure of HSA was altered in the presence of Dhc and DhcA. Furthermore, the atomic force microscopy and transmission electron microscopy showed that the dimensions like height and molecular size of the HSA-Dhc and HSA-DhcA complex were larger compared to HSA alone. Detailed analysis through molecular dynamics simulations also supported greater stability of HSA-Dhc and HSA-DhcA complexes, and root-mean-square-fluctuation interpreted the binding site of Dhc as domain IB and domain IIA for DhcA. This information is valuable for further development of steroid derivative with improved pharmacological significance as novel anti-cancer drugs.

Analysis of Hormone-Protein Binding in Solution by Ultrafast Affinity Extraction: Interactions of Testosterone with Human Serum Albumin and Sex Hormone Binding Globulin

Ultrafast affinity extraction was used to study hormone-protein interactions in solution, using testosterone and its transport proteins human serum albumin (HSA) and sex hormone binding globulin (SHBG) as models. Both single column and two-dimensional systems based on HSA microcolumns were utilized to measure the free fraction of testosterone in hormone/protein mixtures at equilibrium or that were allowed to dissociate for various lengths of time. These data were used to determine the association equilibrium constants (Ka) or global affinities (nKa') and dissociation rate constants (kd) for testosterone with soluble HSA and SHBG. This method was also used to measure simultaneously the free fraction of testosterone and its equilibrium constants with both these proteins in physiological mixtures of these agents. The kd and Ka values obtained for HSA were 2.1-2.2 s(-1) and 3.2-3.5 × 10(4) M(-1) at pH 7.4 and 37 °C. The corresponding constants for SHBG were 0.053-0.058 s(-1) and 0.7-1.2 × 10(9) M(-1). All of these results gave good agreement with literature values, indicating that this approach could provide information on a wide range of rate constants and binding strengths for hormone-protein interactions in solution and at clinically relevant concentrations. The same method could be extended to alternative hormone-protein systems or other solutes and binding agents.

Quantitative Systems Pharmacology Approaches Applied to Microphysiological Systems (MPS): Data Interpretation and Multi-MPS Integration

Our goal in developing Microphysiological Systems (MPS) technology is to provide an improved approach for more predictive preclinical drug discovery via a highly integrated experimental/computational paradigm. Success will require quantitative characterization of MPSs and mechanistic analysis of experimental findings sufficient to translate resulting insights from in vitro to in vivo. We describe herein a systems pharmacology approach to MPS development and utilization that incorporates more mechanistic detail than traditional pharmacokinetic/pharmacodynamic (PK/PD) models. A series of studies illustrates diverse facets of our approach. First, we demonstrate two case studies: a PK data analysis and an inflammation response--focused on a single MPS, the liver/immune MPS. Building on the single MPS modeling, a theoretical investigation of a four-MPS interactome then provides a quantitative way to consider several pharmacological concepts such as absorption, distribution, metabolism, and excretion in the design of multi-MPS interactome operation and experiments.

Reversible increase in maximal cortisol secretion rate in septic shock

OBJECTIVE

Cortisol clearance is reduced in sepsis and may contribute to the development of impaired adrenocortical function that is thought to contribute to the pathophysiology of critical illness-related corticosteroid insufficiency. We sought to assess adrenocortical function using computer-assisted numerical modeling methodology to characterize and compare maximal cortisol secretion rate and free cortisol half-life in septic shock, sepsis, and healthy control subjects.

DESIGN

Post hoc analysis of previously published total cortisol, free cortisol, corticosteroid-binding globulin, and albumin concentration data.

SETTING

Single academic medical center.

PATIENTS

Subjects included septic shock (n = 45), sepsis (n = 25), and healthy controls (n = 10).

INTERVENTIONS

I.v. cosyntropin (250 μg).

MEASUREMENTS AND MAIN RESULTS

Solutions for maximal cortisol secretion rate and free cortisol half-life were obtained by least squares solution of simultaneous, nonlinear differential equations that account for free cortisol appearance and elimination as well as reversible binding to corticosteroid-binding globulin and albumin. Maximal cortisol secretion rate was significantly greater in septic shock (0.83 nM/s [0.44, 1.58 nM/s] reported as median [lower quartile, upper quartile]) compared with sepsis (0.51 nM/s [0.36, 0.62 nM/s]; p = 0.007) and controls (0.49 nM/s [0.42, 0.62 nM/s]; p = 0.04). The variance of maximal cortisol secretion rate in septic shock was also greater than that of sepsis or control groups (F test, p < 0.001). Free cortisol half-life was significantly increased in septic shock (4.6 min [2.2, 6.3 min]) and sepsis (3.0 min [2.3, 4.8 min] when compared with controls (2.0 min [1.2, 2.6 min]) (both p < 0.004).

CONCLUSIONS

Results obtained by numerical modeling are consistent with comparable measures obtained by the gold standard stable isotope dilution method. Septic shock is associated with generally not only higher levels but also greater variance of maximal cortisol secretion rate when compared with control and sepsis groups. Additional studies would be needed to determine whether assessment of cortisol kinetic parameters such as maximal cortisol secretion rate and free cortisol half-life is useful in the diagnosis or management of critical illness-related corticosteroid insufficiency.

Higher free serum cortisol is associated with worse survival in acute variceal bleeding because of cirrhosis: a prospective study

BACKGROUND AND AIMS

Critical illness-related corticosteroid insufficiency has been reported in acute variceal bleeding (AVB). In cirrhosis, free serum cortisol (FC) is considered optimal to assess adrenal function. Salivary cortisol (SC) is considered a surrogate for FC. We evaluated FC and its prognostic role in AVB.

METHODS

Total serum cortisol, SC, cortisol-binding globulin, and FC (Coolens' formula) were evaluated in AVB (n=38) and in stable cirrhosis (CC) (n=31). A Cox proportional hazards model was evaluated for 6-week survival.

RESULTS

In AVB, the median FC and SC levels were higher with worse liver dysfunction [Child-Pugh (CP) A/B/C: 1.59/2.62/3.26 μg/dl, P=0.019; CPA/B/C: 0.48/0.897/1.81 μg/ml, P<0.001, respectively]. In AVB compared with CC, median total serum cortisol: 24.3 versus 11.6 μg/dl (P<0.001), SC: 0.86 versus 0.407 μg/ml (P<0.001); FC 2.4 versus 0.57 μg/dl (P<0.001). In AVB, 5-day rebleeding was 10.5%, and 6-week and total mortality were 21.1 and 23.7%, respectively. Independent associations with 6-week mortality in AVB were FC at least 3.2 μg/dl (P<0.001), hepatocellular carcinoma (P<0.001), CPC (P<0.001), and early rebleeding (P<0.001). Among patients with normal cortisol-binding globulin (n=14) and albumin (n=31), the factors were hepatocellular carcinoma (P=0.003), CP (P=0.003), and FC (P=0.036). SC was also found to be an independent predictor of 6-week mortality (P<0.001). Area under the curve of FC for predicting 6-week mortality was 0.79.

CONCLUSION

Higher FC is present in cirrhosis with AVB compared with CC and is associated independently with bleeding-related mortality. However, whether high FC solely indicates the severity of illness or whether there is significant adrenal insufficiency cannot be discerned.

Development of a formula for estimating plasma free cortisol concentration from a measured total cortisol concentration when elastase-cleaved and intact corticosteroid binding globulin coexist

Cortisol bound to corticosteroid binding globulin (CBG) contributes up to 90% of the total cortisol concentration in circulation. Therefore, changes in the binding kinetics of cortisol to CBG can potentially impact on the concentration of free cortisol, the only form that is responsible for the physiological function of the hormone. When CBG is cleaved into elastase-cleaved CBG (eCBG) by the activity of neutrophil elastase, its affinity for cortisol is reduced. Therefore, when eCBG coexists with intact CBG (iCBG) in plasma, the calculation of free cortisol concentration based on the formulae that considers only one CBG pool with the same affinity for cortisol may be inappropriate. In this study, we developed in vivo and in vitro models of cortisol partitioning which considers two CBG pools, iCBG and eCBG, with different affinities for cortisol, and deduce a new formula for calculating plasma free cortisol concentration. The formula provides better estimates of free cortisol concentration than previously used formulae when measurements of the concentrations of the two CBG forms are available. The model can also be used to estimate the affinity of CBG and albumin for cortisol in different clinical groups. We found no significant difference in the estimated affinity of CBG and albumin for cortisol in normal, sepsis and septic shock groups, although free cortisol was higher in sepsis and septic shock groups. The in vivo model also demonstrated that the concentration of interstitial free cortisol is increased locally at a site of inflammation where iCBG is cleaved to form eCBG by the activity of elastase released by neutrophils. This supports the argument that the cleavage of iCBG at sites of inflammation leads to more lower-affinity eCBG and may be a mechanism that permits the local concentration of free cortisol to increase at these sites, while allowing basal free cortisol concentrations at other sites to remain unaffected.

Estimation of maximal cortisol secretion rate in healthy humans

CONTEXT

Cortisol secretion is related to ACTH concentration by a sigmoidal dose-response curve, in which high ACTH concentrations drive maximal cortisol secretion rates (CSR(max)).

OBJECTIVE

We sought to estimate CSR(max) and free cortisol half-life in healthy humans (n = 21) using numerical methods applied to data acquired during cosyntropin (250 μg) stimulation. We also evaluated the effect of overnight dexamethasone (DEX; 1 mg) vs. placebo on estimates of CSR(max) and free cortisol half-life.

DESIGN

This study was a double-blind, placebo-controlled, randomized order of overnight DEX vs. placebo, cosyntropin (250 μg) stimulation with frequent serum cortisol sampling and computer-assisted numerical analysis.

SETTING

The study was conducted at a single academic medical center.

PARTICIPANTS

Twenty-one healthy adult subjects (15 females and six males), mean aged 46 yr, participated in the study.

INTERVENTION

Intervention in the study included DEX vs. placebo pretreatment, cosyntropin (250 μg) iv with frequent cortisol sampling.

MAIN OUTCOME MEASURES

CSR(max) and free cortisol half-life estimates, R² for goodness of fit, were measured.

RESULTS

Mean ± sd CSR(max) was 0.44 ± 0.13 nm/second, with free cortisol half-life of 2.2 ± 1.1 min. DEX did not significantly affect estimates of CSR(max) or free cortisol half-life. Our model accounts for most of the variability of measured cortisol concentrations (overall R² = 90.9 ±11.0%) and was more accurate (P = 0.004) during DEX suppression (R² = 94.6 ± 4.6%) compared with placebo (R² = 87.2 ± 8.7%).

CONCLUSIONS

Application of a mass-action model under conditions of cosyntropin stimulation provides a relatively simple method for estimation CSR(max) that accurately predicts measured cortisol concentrations. DEX administration did not significantly affect estimates of CSR(max) or free cortisol half-life.

Effect of sex and prior exposure to a cafeteria diet on the distribution of sex hormones between plasma and blood cells

It is generally assumed that steroid hormones are carried in the blood free and/or bound to plasma proteins. We investigated whether blood cells were also able to bind/carry sex-related hormones: estrone, estradiol, DHEA and testosterone. Wistar male and female rats were fed a cafeteria diet for 30 days, which induced overweight. The rats were fed the standard rat diet for 15 additional days to minimize the immediate effects of excess ingested energy. Controls were always kept on standard diet. After the rats were killed, their blood was used for 1) measuring plasma hormone levels, 2) determining the binding of labeled hormones to washed red blood cells (RBC), 3) incubating whole blood with labeled hormones and determining the distribution of label between plasma and packed cells, discounting the trapped plasma volume, 4) determining free plasma hormone using labeled hormones, both through membrane ultrafiltration and dextran-charcoal removal. The results were computed individually for each rat. Cells retained up to 32% estrone, and down to 10% of testosterone, with marked differences due to sex and diet (the latter only for estrogens, not for DHEA and testosterone). Sex and diet also affected the concentrations of all hormones, with no significant diet effects for estradiol and DHEA, but with considerable interaction between both factors. Binding to RBC was non-specific for all hormones. Estrogen distribution in plasma compartments was affected by sex and diet. In conclusion: a) there is a large non-specific RBC-carried compartment for estrone, estradiol, DHEA and testosterone deeply affected by sex; b) Prior exposure to a cafeteria (hyperlipidic) diet induced hormone distribution changes, affected by sex, which hint at sex-related structural differences in RBC membranes; c) We postulate that the RBC compartment may contribute to maintain free (i.e., fully active) sex hormone levels in a way similar to plasma proteins non-specific binding.

Sex differences on the judgment of line orientation task: a function of landmark presence and hormonal status

It has been well-established that men outperform women on some spatial tasks. The tools commonly used to demonstrate this difference (e.g. The Mental Rotations Task) typically involve problems and solutions that are presented in a context devoid of referents. The study presented here assessed whether the addition of referents (or "landmarks") would attenuate the well-established sex difference on the judgment of line orientation task (JLOT). Three versions of the JLOT were presented in a within design. The first iteration contained the original JLOT (JLOT 1). JLOT 2 contained three "landmarks" or referents and JLOT 3 contained only one landmark. The sex difference on JLOT 1 was completely negated by the addition of three landmarks on JLOT 2 or the addition of one landmark on JLOT3. In addition, salivary testosterone was measured. In men, gains in performance on the JLOT due to the addition of landmarks were positively correlated with testosterone levels. This suggests that men with the highest testosterone levels benefited the most from the addition of landmarks. These data help to highlight different strategies used by men and women to solve spatial tasks.

Assessment of adrenal function in cirrhotic patients: salivary cortisol should be preferred

BACKGROUND & AIMS

Adrenal insufficiency is a common disorder among cirrhotic patients. Adrenal function is usually assessed with serum total cortisol assays. Free cortisol (active fraction) represents only 10% of serum total cortisol, the remaining 90% being linked to cortisol-binding globulin (CBG) and albumin. In cirrhotic patients, the synthesis of these proteins is reduced, which could lead to an overestimation of the prevalence of adrenal insufficiency. Salivary cortisol assessment adequately reflects free cortisol plasma concentration. However, this method has never been validated in cirrhotic patients. The objectives of this report were to assess the following parameters by a prospective observational study: (1) correlation between salivary, serum total and free cortisol, (2) adrenal insufficiency prevalence using salivary and serum assays, (3) parameters associated with a discrepancy between both tests, and (4) adrenal insufficiency risk factors among cirrhotic patients.

METHODS

Salivary and serum total cortisol were assessed before and 1h following an injection of corticotropin (250 microg) in patients hospitalized for cirrhosis complications without shock. CBG was measured and free cortisol was assessed by the Coolens formula.

RESULTS

Eighty-eight patients were included in the study (Child-Pugh C: 68.2%). Free cortisol was more strongly correlated with salivary than with serum total cortisol (Spearman coefficient=0.91 vs. 0.76, respectively, p<0.001). Among included patients, 9.1% had adrenal insufficiency according to salivary cortisol and 33.0% had adrenal insufficiency according to serum total cortisol (p=0.001). Hypoalbuminemia was the only factor associated with a discrepancy between the results of both tests. Adrenal insufficiency risk factors were ascites and low HDL-cholesterol plasma concentration.

CONCLUSION

Using serum total cortisol assays overstate adrenal insufficiency prevalence among cirrhotic patients, mainly because of inaccurate concentrations related to hypoalbuminemia. Salivary cortisol assays should be preferably used in these patients.

Physiological concentrations of albumin stimulate chorionic gonadotrophin and placental lactogen release from human term placental explants

This study investigates whether albumin, a major plasma protein in direct contact with the trophoblast in vivo, can modulate human chorionic gonadotrophin (HCG) and human placental lactogen (HPL) releases from placental explants. Incubating explants with a near physiological, i.e. 5%, concentration of human or bovine albumin during 30 min increased HCG and HPL release by at least 150%. This albumin effect was not mediated by any difference in hormone adsorption onto glass surfaces. In contrast to the sustained stimulation of hormone releases elicited by the addition of 10 mmol/l extracellular calcium, the albumin-mediated secretory responses were transient. However, the albumin- and calcium-stimulatory effects were abolished at 4 degrees C, depressed by 0.36 mmol/l cycloheximide or 1 mmol/l colchicine and potentiated by 40 micromol/l cytochalasin B. Moreover, the stimulatory effect of albumin on the hormone releases was not modified in the absence of Ca(2+) or in the presence of 1 or 10 mmol/l Ca(2+) in the extracellular milieu. These data suggest that albumin is involved, at physiological concentration, in the secretion of HCG and HPL by human placenta. The cellular mechanism(s) underlying the albumin-mediated secretory responses may be partly different from those involved during the calcium-mediated stimulation.

The evolution of gene expression, structure and function of transthyretin

Thyroxine, the most abundant thyroid hormone in blood, partitions into lipid membranes. In a network-like system, thyroxine-binding plasma proteins counteract this partitioning and establish intravascular, protein-bound thyroxine pools. These are far larger than the free thyroxine pools. In larger eutherians, proteins specifically binding thyroxine are albumin, transthyretin, and thyroxine-binding globulin. Some binding of thyroxine can also occur to lipoproteins. During evolution, transthyretin synthesis first appeared in the choroid plexus of the stem reptiles, about 300 million years ago. Transthretin synthesis in the liver evolved much later, independently, in birds, eutherians and some marsupial species. Analysis of 57 human transthyretin variants suggests that most mutations in transthyretin are not compatible with its normal metabolism and lead to its deposition as amyloid. Analysis of transthyretin or its gene in 20 different species shows that evolutionary changes of transthyretin predominantly occurred near the N-termini. A change in RNA splicing between exon 1 and exon 2 led to a decrease in hydrophobicity and length of the N-termini. It is proposed that the selection pressure producing these changes was the need for a more effective prevention of thyroxine partitioning into lipids. Lipid pools increased during evolution with the increases in relative sizes of brains and internal organs and changes in lipid composition of membranes in ectothermic and endothermic species.

The effects of sex hormone binding globulin (SHBG) on testosterone transport into the cerebrospinal fluid

The movement of testosterone (T) from blood across the blood-brain barrier (BBB) is thought to reflect the combined effects of T's lipid solubility and the presence of circulating binding proteins for T such as albumin or sex hormone binding globulin (SHBG). Since the adult rat lacks a circulating specific high affinity sex steroid binding protein, examination of the disappearance from serum and uptake into cerebrospinal fluid (CSF) of [3H]T before and after SHBG or albumin infusion should provide insight into the function of these two proteins with respect T transport. Three groups of adult male Sprague-Dawley rats were cannulated at the femoral vein and cisterna magna. In a control group (n = 8), [3H]T was given as an intravenous bolus beginning at time zero; multiple serum and CSF collections were assayed for counts per min (cpm) during the subsequent 45 min. Data from these animals were then compared to those seen in animals that received either purified human SHBG (hSHBG) (n = 7) or human albumin (hALB) (n = 6) 10 min prior to the [3H]T infusion. High performance liquid chromatography was used to monitor the metabolic fate of the steroid infusate at the end of each study period. Infusion of hSHBG increased serum concentrations from undetectable to 93.8 nM/l (mean +/- SEM, n = 6). Administration of hALB significantly increased (25.0 +/- 1.2 g/l at baseline, 33.4 +/- 1.6 g/l post-infusion, mean +/- SEM, P less than 0.03, n = 5) the circulating albumin concentration. Comparison of data from each group of animals demonstrated that (1) following an i.v. injection of radiolabeled T, the initial decline in serum [3H]T was significantly reduced (P less than 0.03) in the presence of hSHBG, (2) hALB did not affect the movement of [3H]T out of serum, (3) the time to peak appearance of [3H]T in the CSF was significantly delayed (P less than 0.02) by the presence of circulating hSHBG, and (4) the net quantity of [3H]T found in the cSF under steady-state conditions was not affected by serum SHBG or albumin levels. This study demonstrates that high-affinity steroid binding proteins do modulate the transport of sex steroids across the BBB. Specifically, SHBG delays the clearance of T from serum and slows the rate of T uptake into the CSF during non-equilibrium conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Rates of dissociation of sex steroid hormones from human sex hormone-binding globulin: a reassessment

A rapid filtration assay employing dextran-coated charcoal as acceptor particles for free hormone was used to measure the rates of dissociation of dihydrotestosterone (DHT), testosterone (T), and estradiol (E2) from their binding proteins in human serum at 37 degrees C. Because measurements were begun after each hormone had fully (greater than 99%) dissociated from albumin, the observed rates of dissociation correspond to the rates of dissociation of the sex hormone-binding globulin (SHBG)-hormone complexes. The dissociation rate constants of the hormone-SHBG complexes were determined to be 0.016 +/- 0.001, 0.056 +/- 0.002, and 0.083 +/- 0.003 s-1 for DHT, T, and E2, respectively, corresponding to half-times of dissociation (t1/2) of 43, 12 and 8.4 s, respectively. The physiological significance of these findings can best be appreciated by comparing these t1/2 s with the capillary and sinusoidal transit times of various tissues (less than 1 s to approximately 10 s).