Microperimetry as a diagnostic tool for the detection of early, subclinical retinal damage and visual impairment in multiple sclerosis.

BACKGROUND: A majority of multiple sclerosis patients experience visual impairment, often as the initial presenting symptom of the disease. While structural changes in the retinal nerve fiber layer and optic nerve have demonstrated correlations with brain atrophy in multiple sclerosis using magnetic resonance imaging, a non-invasive, cost-effective, and clinically efficacious modality to identify early damage and facilitate prompt therapeutic intervention to slow the progression of multiple sclerosis and its ocular manifestations, is still urgently needed. In this study, we sought to determine the role of macular sensitivity measured by microperimetry in the detection of subclinical multiple sclerosis-related retinal damage and visual dysfunction. METHODS: This cross-sectional observational case-control study involved population-based samples of multiple sclerosis patients and age-, race-, and gender-matched healthy control subjects. Among the key criteria for the multiple sclerosis patients were diagnosis by the McDonald criteria, visual acuity greater than 20/25, and no history of optic neuritis. Macular sensitivity and average macular thickness were measured in all subjects using microperimetry and spectral-domain optical coherence tomography, respectively. Pearson correlation coefficients were measured using bivariate correlations. Sample means, mean differences, and 95% confidence intervals were calculated using independent sample t-tests. RESULTS: Twenty-eight eyes from 14 MS patients and 18 eyes from 9 control subjects were included. Mean macular sensitivity of control subjects and multiple sclerosis patients in decibels was 18.2 ± 0.4 and 16.5 ± 0.4, respectively, corresponding to a mean difference of 1.7 (95% CI, 1.1-2.4; P < 0.001). Macular sensitivity was positively correlated with macular thickness in multiple sclerosis patients (r = 0.49, P = 0.01) but not control subjects (r = 0.15, P = 0.55). CONCLUSIONS: Macular sensitivity as measured by microperimetry was decreased in multiple sclerosis patients with normal visual acuity and no history of optic neuritis. Furthermore, macular sensitivity demonstrated a positive correlation with macular thickness as measured by optical coherence tomography. As such, microperimetry may represent a non-invasive and efficient method to identify signs of subclinical visual dysfunction that correspond with early macular architectural changes characteristic of multiple sclerosis.

Anandamide regulates lipopolysaccharide-induced nitric oxide synthesis and tissue damage in the murine uterus.

In women, the association between chronic marijuana smoking and early miscarriage has long been known. Anandamide, a major endocannabinoid, mimics some of the psychotropic, hypnotic and analgesic effects of Delta(9)-tetrahydrocannabinol, the psychoactive component of marijuana. The uterus contains the highest concentrations of anandamide yet discovered in mammalian tissues and this suggests that it might play a role in reproduction. The production of small amounts of nitric oxide (NO) regulates various physiological events including implantation and myometrial relaxation, but in an inflammatory setting such as sepsis, NO has toxic effects as it is a free radical. The results presented in this study indicate that anandamide modulates NO production induced by lipopolysaccharide (LPS) in an in-vitro murine model. It was shown that LPS-induced NO synthesis and tissue damage were mediated by anandamide, as a cannabinoid receptor type I antagonist could block the effect of LPS (P < 0.001). This endotoxin inhibited anandamide uterine degradation (P < 0.05) and increased the expression of one of its synthesizing enzymes (P < 0.05). Contrary to the known anti-inflammatory and protective effects, in this model anandamide seems to act as a pro-inflammatory molecule modulating the production of NO induced by LPS. This proinflammatory effect of anandamide may be implicated in pathological reproductive events such as septic abortion.

Endocannabinoid system and nitric oxide are involved in the deleterious effects of lipopolysaccharide on murine decidua.

Endocannabinoids are an important family of lipid-signaling molecules that are widely distributed in mammalian tissues and anandamide (AEA) was the first member identified. The uterus contains the highest concentrations of AEA yet discovered in mammalian tissues and this suggests that it might play a role in reproduction. Previous results from our laboratory have shown that AEA modulated NO synthesis in rat placenta. The production of small amounts of nitric oxide regulates various physiological reproductive processes such as implantation, decidualization and myometrial relaxation. But in an inflammatory setting such as sepsis, NO is produced in big amounts and has toxic effects as it is a free radical. The results presented in this study indicate that LPS-induced NO synthesis and tissue damage were mediated by AEA. Decidual LPS-induced NO production was abrogated either by co-incubation with CB1 (AM251) or CB2 (SR144528) antagonists which suggests that both receptors could be mediating this effect. On the other hand, LPS-induced tissue damage and this deleterious effect was partially abrogated by incubating tissue explants with LPS plus CB1 receptor antagonist. Our findings suggest that AEA, probably by increasing NO synthesis, participates in the deleterious effect of LPS in implantation sites. These effects could be involved in pathological reproductive events such as septic abortion.

Spasmus nutans-like nystagmus is often associated with underlying ocular, intracranial, or systemic abnormalities.

BACKGROUND: There is uncertainty as to whether spasmus nutans (SN) is an isolated idiopathic entity or whether there are underlying conditions that could cause or be associated with the nystagmus. We undertook this study to determine the frequency of ocular, intracranial, and systemic conditions in patients with nystagmus having characteristics of SN. METHODS: We performed a chart review of 22 consecutive patients examined from 2000 through 2005 at the Dean McGee Eye Institute and Children' Hospital of Oklahoma with nystagmus consistent with SN. We collected information related to gender, age at presentation and age at final visit, visual acuity, refractive error, laterality of nystagmus, presence of head nodding and torticollis, pattern of strabismus, neuroimaging and electroretinography results, and other associated clinical findings. RESULTS: Visual acuity was reduced in 75% of eyes at presentation and 58% of eyes at last visit. Eight patients had significant refractive error. Seven patients had strabismus. Two patients had chiasmal gliomas. Four patients had cone or rod/cone dystrophy. Only three patients had no associated ocular, intracranial, or systemic conditions. CONCLUSIONS: A substantial proportion of patients presenting with SN-like nystagmus have important underlying ocular, intracranial, or systemic abnormalities that may require evaluation and management.

Nitric oxide mediates prostaglandins' deleterious effect on lipopolysaccharide-triggered murine fetal resorption.

Genital tract bacterial infections could induce abortion and are some of the most common complications of pregnancy; however, the mechanisms remain unclear. We investigated the role of prostaglandins (PGs) in the mechanism of bacterial lipopolysaccharide (LPS)-induced pregnancy loss in a mouse model, and we hypothesized that PGs might play a central role in this action. LPS increased PG production in the uterus and decidua from early pregnant mice and stimulated cyclooxygenase (COX)-II mRNA and protein expression in the decidua but not in the uterus. We also observed that COX inhibitors prevented embryonic resorption (ER). To study the possible interaction between nitric oxide (NO) and PGs, we administered aminoguanidine, an inducible NO synthase inhibitor. NO inhibited basal PGE and PGF(2alpha) production in the decidua but activated their uterine synthesis and COX-II mRNA expression under septic conditions. A NO donor (S-nitroso-N-acetylpenicillamine) produced 100% ER and increased PG levels in the uterus and decidua. LPS-stimulated protein nitration was higher in the uterus than in the decidua. Quercetin, a peroxynitrite scavenger, did not reverse LPS-induced ER. Our results suggest that in a model of septic abortion characterized by increased PG levels, NO might nitrate and thus inhibit COX catalytic activity. ER prevention by COX inhibitors adds a possible clinical application to early pregnancy complications due to infections.

Nitric oxide (NO) inhibits prostaglandin E2 9-ketoreductase (9-KPR) activity in human fetal membranes.

Nitric oxide (NO) synthesized by fetal membranes may act either directly inhibiting myometrium contractility or indirectly interacting with tocolytic agents as prostaglandins (PGs). Here we examined if NO could modulate prostaglandin E(2) 9-ketoreductase (9-KPR) activity in human fetal membranes (HFM). 9-KPR is the enzyme that converts PGE(2) into PGF(2alpha), the main PGs known to induce uterine contractility at term. Chorioamnion explants obtained from elective caesareans were incubated with aminoguanidine (AG), an iNOS inhibitor, or NOC-18, a NO donor. NOC-18 (2mM) increased PGE(2) production and diminished PGF(2alpha) synthesis in HFM. AG presented the opposite effect. When we evaluated the activity of 9-KPR by the conversion of [(3)H]-PGE(2) into [(3)H]-PGF(2alpha) and 13,14-dihidro-15-keto prostaglandin F(2alpha) (the PGF(2alpha) metabolite), we found that NOC-18 inhibited 9-KPR activity. Interestingly, AG did not elicit any effect on 9-KPR but l-NAME, a non-selective NOS inhibitor, significantly increased its activity. Our data suggests that exogenous NO inhibits 9-KPR activity in HFM, thus modulating the synthesis of important labor mediators as PGF(2alpha).

Effect of in vivo administration of epidermal growth factor on prostaglandin production and NOS activity in term rat placentae. Possible participation of placental EGF receptors.

Many authors hypothesize that the epidermal growth factor (EGF) is involved in the onset of labor. Previous reports from our laboratory showed that intrauterine administration of EGF delays the beginning of labor. The aims of this study were: 1) to analyze the effect of intrauterine administration of 500 ng EGF on placental prostaglandins and nitric oxide, and 2) to characterize the expression of EGF receptors (EGF-R) in pregnant rat placentae. Saline solution (sham group) and 500 ng EGF (EGF-treated group) were administered via intrauterine injection on day 21 of gestation, and both groups of animals were sacrificed on day 22 (sham rats delivered on day 22). Results showed that EGF treatment: 1) inhibited the production of prostaglandin E (p<0.001) and F(2alpha) (p<0.01), 2) increased the synthesis of nitric oxide (p<0.001), and 3) reduced the expression of cyclooxygenase-II, the enzyme responsible for PG synthesis. Placentae were found to express EGF-R and its activated form, and the expressions of both forms were higher at mid and term pregnancy. Hence, EGF is a very interesting molecule for studying the regulation of placental prostaglandin and nitric oxide production related to the parturition process.

Epidermal growth factor prevents prepartum luteolysis in the rat.

We have previously reported that intrauterine (i/u) administration of epidermal growth factor (EGF 500 ng) on day (d) 21 of pregnancy delayed 19.0 +/- 0.6 h the onset of labor. Progesterone (P) is secreted by ovarian corpora lutea (CL) throughout gestation in the rat. Prepartum CL regression due to increased uterine cyclooxygenase I and prostaglandin F(2alpha) results in P withdrawal followed by labor. The aims of the present work were (i) to study whether EGF delayed-onset of labor was mediated by a mechanism that prevented CL regression; (ii) to determine amniotic fluid (AF) EGF in pregnant rats. Rats on d21 of pregnancy received i/u EGF (500 ng) and were killed 0, 4, 8, 12, 24, and 48 h later. Control AF from rats on d13 and 18-22 of pregnancy was obtained. EGF decreased uterine prostaglandin F(2alpha) synthesis 8 h after treatment. Twelve hours after EGF injection, P reached its highest serum level and uterine cyclooxygenase I expression was undetectable. CL from rats killed 8 and 12 h after EGF were similar to those from rats on d13 of pregnancy, when serum P is maximum. EGF in AF increased throughout gestation, reached a maximum on d21, and decreased before the onset of labor. We suggest that the effect of EGF on the onset of labor was mediated by an early effect on the uterus that prevented prepartum CL regression.

Biosynthesis and catabolism of prostaglandin F2alpha (PGF2alpha) are controlled by progesterone in the rat uterus during pregnancy.

Myometrial quiescence is a key factor in all species to accomplish a successful gestation. PGs play a crucial role in mediating parturition events, and their synthesis and metabolism are regulated by cyclooxygenases (COXs) and NAD(+)-dependent 15-hydroxy-PG dehydrogenase (PGDH), respectively. Progesterone (P(4)) is the hormone responsible for maintaining uterine smooth muscle quiescence during pregnancy. In this work, we have studied the effect of P(4) on the activity of COXs and PGDH, the uterine enzymes involved in the biosynthesis and metabolism of prostanoids in the rat. We found that during pregnancy PGF(2alpha) production and also protein levels of COX-1 and COX-2 were decreased. The exogenous administration of P(4) significantly inhibited the uterine production of PGF(2alpha) and also the protein level of COX-2. PGF(2alpha), metabolism was assessed by PGDH activity, which resulted high during pregnancy and increased as a result of P(4) administration. These results indicate that PGs levels were negatively modulated by P(4), which could be exerting its effect by increasing PGs metabolism through stimulation on PGDH activity and an inhibition on COX and that is a major mechanism for maintain uterine quiescence in pregnancy.

[Evaluation of deferoxamine capacity to revert a severe porphyria induced by hexachlorobenzene]. / Evaluación de la capacidad de la desferrioxamina para revertir una severa porfiria inducida por hexaclorobenceno.

The purpose of the present work is to investigate the ability of desferrioxamine (DF) as an iron chelator to revert or decrease a severe experimental porphyria induced by hexachlorobenzene (HCB) in rats; DF treatment started after 17 weeks of HCB intoxication and was continued until the 27th week. The urinary excretions of -aminolevulinic acid (ALA), porphobilinogen and porphyrins were weekly quantitated. At the end of the experiment the animals were sacrificed and hepatic porphyrins, ALA-synthase and porphyrinogen carboxy-lyase activities were determined. The results obtained indicated that, under the present conditions, the administered iron chelator does not improve the disturbance promoted by HCB on the haem pathway. These results were compared with those obtained when the DF was given simultaneously with HCB from the beginning of fungicide administration. In this last situation the chelator was able to delay and diminish the porphyrinogenic effect of HCB.

Evaluación de la capacidad de la desferrioxamina para revertir una severa porfiria inducida por hexaclorobenceno. / [Evaluation of deferoxamine capacity to revert a severe porphyria induced by hexachlorobenzene]

The purpose of the present work is to investigate the ability of desferrioxamine (DF) as an iron chelator to revert or decrease a severe experimental porphyria induced by hexachlorobenzene (HCB) in rats; DF treatment started after 17 weeks of HCB intoxication and was continued until the 27th week. The urinary excretions of -aminolevulinic acid (ALA), porphobilinogen and porphyrins were weekly quantitated. At the end of the experiment the animals were sacrificed and hepatic porphyrins, ALA-synthase and porphyrinogen carboxy-lyase activities were determined. The results obtained indicated that, under the present conditions, the administered iron chelator does not improve the disturbance promoted by HCB on the haem pathway. These results were compared with those obtained when the DF was given simultaneously with HCB from the beginning of fungicide administration. In this last situation the chelator was able to delay and diminish the porphyrinogenic effect of HCB.

Effect of desferrioxamine on the development of hexachlorobenzene-induced porphyria.

The present work deals with the effect of desferrioxamine (DF) on hexachlorobenzene (HCB)-induced porphyria in female rats with the purpose of further investigation of the role of iron in the development of this porphyria. The results obtained show that the repeated injection of DF (three times a week: 100 mg/kg each i.m.) delayed and diminished remarkably the urinary excretion of precursors and porphyrins as well as the accumulation of the latter in liver promoted by HCB (1 g/kg daily given by stomach tube). This was probably due to attenuation by DF of the alterations produced by the fungicide in the two key enzymes: porphyrinogen carboxy-lyase (PCL) and delta-aminolaevulinate synthase (ALA-S). In fact, DF by reducing liver iron levels produced a smaller decrease of the target enzyme (PCL) and a concomitant smaller induction of ALA-S. DF alone did not modify any of these variables or the liver to body weight ratio. DF added at 10(-2) and 10(-3) M to the incubation media of ALA-S and PCL did not alter either of the enzymatic activities, whether in normal or HCB-porphyric preparations. The results obtained show that DF improved the biochemical picture during HCB porphyria. They suggest that iron plays an indirect role in the decrease of PCL enzyme, possibly at the HCB metabolization step. A common iron-involving mechanism for the production of porphyria by different chlorinated compounds is suggested.

Screening for the ability of hexachlorobenzene metabolites to decrease rat liver porphyrinogen carboxy-lyase.

In order to examine inhibitory effects of hexachlorobenzene metabolites on the hepatic porphyrinogen carboxylyase activity, rat liver cytosol was incubated with uroporphyrinogen III and chlorinated phenols, thiophenols, thioanisoles and benzenes. Then, the occurrence of hepta-, hexa-, penta- and tetracarboxyporphyrinogen = coproporphyrinogen (measured as porphyrins) was determined. Inhibitory effects were exerted by tetrachlorohydroquinone, pentachlorophenol, pentachlorothiophenol, 1,2,3,5- and 1,2,4,5-tetrachlorobenzene. Other compounds including hexachlorobenzene which was tested for comparative reasons did not impair uroporphyrinogen decarboxylation. In the presence of tetrachlorohydroquinone, uroporphyrinogen merely was decarboxylated to hepta- and hexacarboxyporphyrinogen. Under the influence of the other 4 compounds with inhibitory effects, pentacarboxyporphyrinogen and coproporphyrinogen were formed additionally. Coproporphyrinogen formation was inhibited completely by tetrachlorohydroquinone, while pentachlorophenol diminished its formation by about 50% and pentachlorothiophenol, 1,2,3,5- and 1,2,4,5-tetrachlorobenzene by less than 10%.

Ability of several hexachlorobenzene metabolites to decrease rat-liver porphyrinogen carboxy-lyase and to produce porphyrin accumulation in chick-embryo liver.

Since hexachlorobenzene (HCB) action seems to be mediated through its metabolites, this study aimed to identify the metabolites and account for the HCB action by evaluating the ability of several of them to inhibit normal rat-liver porphyrinogen carboxy-lyase in vitro and to produce porphyrin accumulation in chick-embryo liver in ovo. Only three of the 11 metabolites assayed produced significant inhibitory effects at 10(-3) M concentration, the order being tetrachlorohydroquinone greater than pentachlorophenol much greater than pentachlorothiophenol. At concentrations below 10(-4) M tetrachlorohydroquinone did not inhibit the enzyme. Most of the metabolites assayed produced porphyrin accumulation, in the following order of strength: chlorophenols greater than chlorothiophenols greater than chlorobenzenes. Phenolic metabolites, therefore, not only produce the greatest amounts of porphyrin accumulation in the liver of the chick embryo but are also the strongest of the metabolite inhibitors of porphyrinogen carboxy-lyase in rat liver. It is possible that they decrease enzymatic activity by binding to the enzyme. This paper discusses the implications of these results for the mechanism of HCB porphyria induction.

Does feeding of hexachlorobenzene promote structural changes in rat-liver porphyrinogen carboxy-lyase?

This study aimed to determine whether drug treatment produced structural changes in porphyrinogen carboxy-lyase enzymatic protein, leading to altered properties. Rat-liver enzyme was obtained from normal animals and from those with hexachlorobenzene (HCB)-induced porphyria, and several of its properties were comparatively studied. The enzymes from both sources were purified 110-fold. They were similar in subcellular distribution, ammonium sulfate fractionation, calcium phosphate gel adsorption, storage stability, requirements of incubation media, effects of salts and photo-oxidizing agents. The enzymes differed with respect to effect of incubation temperature, pH, aerobiosis, chelating agents, dithiothreitol, methylene blue, heat stability and chromatographic behaviour on DEAE-cellulose and Sephadex G-100. These differences would appear to indicate structural differences between the two types of enzymatic protein. Since the porphyrinogenic action of HCB can be mediated through a metabolite, structural differences could arise through the binding of an inhibitory metabolite to the whole enzyme or through modifications to the protein during its synthesis.

Rat-liver porphyrinogen carboxy-lyase inhibition as a function of the degree of hexachlorobenzene-induced porphyria.

This study aimed to confirm the presence of an inhibitor in the hexachlorobenzene-porphyric liver that is able to decrease the normal activity of porphyrinogen carboxy-lyase (PCL), to determine whether any relation exists between the degree of hexachlorobenzene-induced porphyria and the ability of a porphyric liver preparation to reduce the enzyme activity of normal liver and to seek extraction methods in order to characterize the inhibitor by gas-liquid chromatography. A perfused liver supernatant (11,000 X g) filtered through a Sephadex G-25 column and heated for 5 min at 100 degrees C was used as the inhibitor source. The results show that the inhibitor was eluted together with a protein peak by gel filtration, the inhibitor was thermostable, the extent of the inhibitory effect reached by this preparation increased with the degree of porphyria, ether and toluene extracts from both heated and non-heated porphyric liver preparations also exhibited an inhibitory effect on the normal activity of PCL and the degree of inhibition depended on the amount of the preparation added. Therefore, there is an inhibitor of PCL activity in the hexachlorobenzene-porphyric liver, the concentration of which increases as the degree of porphyria increases. This inhibitor is soluble in organic solvents and is presently being characterized by gas-liquid chromatography.

Effect of an iron chelator on the onset of hexachlorobenzene-induced porphyria in rats.

The effect of desferrioxamine on hexachlorobenzene (HCB)-induced porphyria was studied in female rats in order to investigate the role of iron in the development of this porphyria. Repeated injections of desferrioxamine delayed and remarkably diminished the urinary excretion of precursors and porphyrins and the accumulation of porphyrins in the liver. These effects were produced because the desferrioxamine attenuated the alterations produced by HCB in two key enzymes: porphyrinogen carboxy-lyase and delta-aminolaevulinic acid synthase. The effect of desferrioxamine on both enzymes was also studied in vitro. This work showed that iron plays an important role in the onset of HCB-induced porphyria and supplied information on the mechanism of action. A common iron-involving mechanism for the production of porphyria by different chlorinated compounds is suggested.

Ability of several hexachlorobenzene metabolites to induce porphyrin accumulation in chick embryo liver [quot ]in ovo[quot ]

The present study investigates the ability of several hexachlorobenzene (HCB) metabolites to induce porphyrin accumulation in chick embryo liver cells in ovo, in order to further clarify the role of metabolites in the mechanism of HCB-induced porphyria. HCB per se had no effect on liver porphyrin content, but pretreatment assays with phenobarbital suggested that its metabolic products did. When the direct effect of phenolic, sulfur-containing, and benzenic metabolites of HCB were tested, the following results were obtained. Less chlorinated benzenes (pentachlorobenzene and 1,2,3,4- 1,2,3,5- 1,2,4,5- tetrachlorobenzene) had poor capacity to change the control porphyrin content. On the other hand, the behavior of phenolic metabolites (pentachlorophenol, 2,3,4,5- 2,3,4,6- 2,3,5,6- tetrachlorophenol, 2,3,4- 2,3,5- 2,3,6- 2,4,5- 2,4,6- 3,4,5- trichlorophenol and tetrachlorohydroquinone) as porphyrin inducers was remarkable; the stronger effects were produced by trichlorophenols and tetrachlorohydroquinone. Sulfur containing metabolites produced increases in porphyrin content that were lower than those produced by phenolic compounds and higher than those due to the action of less chlorinated benzenes; only 1-methyl-(2,3,4,5-pentachlorophenyl) sulfoxide was not able to increase porphyrin level. The extent of the effect of the other drugs was pentachlorothiophenol greater than 1-methyl-(2,3,4,5,6-pentachlorophenyl) sulfone greater than tetrachlorothioanisol greater than pentachlorothioanisol. Regarding the mechanism of HCB porphyria, the present results indicate that phenolic metabolites and, in a lower degree, sulfur-containing metabolites, can contribute to the porphyrinogenic ability of HCB.

Ability of several hexachlorobenzene metabolites to induce porphyrin accumulation in chick embryo liver "in ovo".

The present study investigates the ability of several hexachlorobenzene (HCB) metabolites to induce porphyrin accumulation in chick embryo liver cells in ovo, in order to further clarify the role of metabolites in the mechanism of HCB-induced porphyria. HCB per se had no effect on liver porphyrin content, but pretreatment assays with phenobarbital suggested that its metabolic products did. When the direct effect of phenolic, sulfur-containing, and benzenic metabolites of HCB were tested, the following results were obtained. Less chlorinated benzenes (pentachlorobenzene and 1,2,3,4- 1,2,3,5- 1,2,4,5- tetrachlorobenzene) had poor capacity to change the control porphyrin content. On the other hand, the behavior of phenolic metabolites (pentachlorophenol, 2,3,4,5- 2,3,4,6- 2,3,5,6- tetrachlorophenol, 2,3,4- 2,3,5- 2,3,6- 2,4,5- 2,4,6- 3,4,5- trichlorophenol and tetrachlorohydroquinone) as porphyrin inducers was remarkable; the stronger effects were produced by trichlorophenols and tetrachlorohydroquinone. Sulfur containing metabolites produced increases in porphyrin content that were lower than those produced by phenolic compounds and higher than those due to the action of less chlorinated benzenes; only 1-methyl-(2,3,4,5-pentachlorophenyl) sulfoxide was not able to increase porphyrin level. The extent of the effect of the other drugs was pentachlorothiophenol greater than 1-methyl-(2,3,4,5,6-pentachlorophenyl) sulfone greater than tetrachlorothioanisol greater than pentachlorothioanisol. Regarding the mechanism of HCB porphyria, the present results indicate that phenolic metabolites and, in a lower degree, sulfur-containing metabolites, can contribute to the porphyrinogenic ability of HCB.