A comparative study of the effects of tetrodotoxin and the removal of external Na+ on the resting potential: evidence of separate pathways for the resting and excitable Na currents in squid axon

To investigate whether the Na permeability of the resting membrane is determined predominantly by the excitable Na channel, we examined the effects of tetrodotoxin (TTX) and the complete removal of external Na+ on the resting potential. In the intact squid axon bathed in K-free artificial seawater, both TTX and the removal of Na+ produced small hyperpolarizations. The effect of Na removal, however, was larger than that of TTX. In the perfused squid axon, the hyperpolarization produced by the removal of external Na+ was greatly enhanced when the internal K concentration ([K+]i) was reduced. The effect of TTX, on the other hand, was not sensitive to the [K+]i or to the membrane potential. For [K+]i = 50 mM and [K+]o = 0, the average hyperpolarization produced by TTX was 1.2 mV, while the hyperpolarization produced by Na removal was approximately 21 mV. The difference between these two effects suggests that the majority of the resting Na current passes through pathways other than the excitable Na channel.

Effect of allopurinol on the utilization of purine degradation pathway intermediates by tobacco cell cultures

Suspension cultured Nicotiana tabacum (tobacco) cells grow slowly on intermediates of the purine degradation pathway (hypoxanthine, xanthine, uric acid, allantoin, and urea) as their sole nitrogen source indicating that this degradation pathway is operative in these cells. The hypoxanthine analog, allopurinol inhibited tobacco cell growth on hypoxanthine but not uric acid. This helps confirm that the site of action of allopurinol is the conversion of hypoxanthine to uric acid by xanthine oxidase. Attempts to select cells which could grow in the presence of allopurinol with hypoxanthine as the nitrogen source were not successful.

Bioavailability of selenium to residents in a low-selenium area of China

For 8 wk 5 groups of 10 men each were given 0.5 g/day DL-methionine, 150 micrograms Se/day as sodium selenite with or without methionine or 150 micrograms Se/day as selenomethionine with or without methionine. Twenty subjects received placebo as controls. Initially plasma Se rose more rapidly than RBC Se. Increases in Se levels were significantly greater with selenomethionine than with the selenite supplement. In the placebo and methionine supplemented groups neither plasma nor RBC Se varied significantly over the course of the study. Supplementation with selenium resulted in marked increases in plasma and RBC GSH-Px within 2 and 4 wk, respectively. Plasma and RBC GSH-Px activity did not differ significantly between Se-supplemented groups. These studies suggest that selenomethionine-Se was more effective in raising plasma and RBC Se than was selenite-Se. Methionine supplements may increase the bioavailability of selenium in severely deficient subjects.

Selenium intake and metabolic balance of 10 men from a low selenium area of China

Selenium intake and urinary and fecal Se excretion of 10 healthy men from a low Se area in China were determined for three consecutive days, in summer, fall, and winter of 1983, and the spring of 1984 while self-selected diets were being consumed. Mean daily Se intake was 8.8 micrograms/day with a range of 2.3-35.5 micrograms/day, and was far below the recommended range of safe and adequate Se intake of 50-200 micrograms Se/day (National Academy of Sciences/National Research Council). Mean urinary and fecal Se outputs were 3.7 and 3.4 micrograms Se/day, respectively. Mean Se balance during this time was +1.8 micrograms Se/day. Apparent absorption of Se approximated 57%. The low Se intake in this area is a cause for concern since the residents of Molimo may be at risk for Se deficiency diseases.

Acute effects of testicular and adrenal cortical blockade on protein synthesis and dihydrotestosterone content of human prostate tissue

To examine the relationship between whole prostatic dihydrotestosterone (DHT) concentrations and epithelial and stromal protein synthesis, patients awaiting surgery for benign prostatic hypertrophy were given medication to reduce circulating and tissue androgen levels for comparison to levels in untreated patients. The medications, either megestrol acetate, a progestational antiandrogen, or megestrol acetate plus dexamethasone, were given for 1 week before surgery. Aliquots of prostate tissue obtained at surgery were assayed for DHT. The remainder of the tissue was separated into stromal and epithelial cells using enzymatic separation; the incorporation of both [3H] proline into stromal protein and [3H]leucine into epithelial cell protein was then measured. Over a wide range of tissue DHT concentrations, DHT correlated significantly with [3H]proline incorporation into stromal protein (r = 0.58). Likewise, epithelial cell incorporation of [3H]leucine into protein correlated significantly with tissue DHT concentrations, with a r value of 0.79. In the case of stromal cells, no additional effects of dexamethasone given with megestrol acetate were found on stromal protein synthesis. Epithelial cell [3H]leucine incorporation into protein was paradoxically enhanced by dexamethasone plus megestrol acetate compared to the latter alone, despite the fact that tissue DHT and circulating adrenal androgens, dehydroepiandrosterone sulfate and delta 4-androstenedione, were not significantly different from values obtained after treatment with megestrol acetate alone. These studies support the conclusion that there is a dominant role of DHT in regulating protein synthesis in both prostatic epithelial and stromal cells. Dexamethasone appears to have a growth-stimulating effect on prostatic epithelial cells.

Presence of oxytocin and arginine vasopressin in human ovary, oviduct, and follicular fluid

Oxytocin and arginine vasopressin (AVP) immunoreactivity in human ovary, oviduct, and follicular fluid were measured and found to coelute with the authentic peptides using both gel filtration column chromatography and reverse phase thin layer chromatography. In ovarian tissue, mean oxytocin and AVP concentrations were 0.48 and 0.24 ng/mg protein, respectively. These values are approximately 4000-fold higher than peripheral plasma levels. The concentration of oxytocin in the corpus luteum was approximately 6-fold greater (3.12 ng/mg protein) than that in ovarian tissue with no corpus luteum. In contrast, no significant difference in the concentration of AVP was found between corpus luteal and the remaining ovarian tissues. Follicular fluid contained 299 and 131 pg/ml oxytocin and AVP, respectively. These levels were 30-fold greater than the serum level of either peptide, suggesting ovarian synthesis of the neurohypophyseal hormones. In addition, immunoreactive oxytocin and AVP were detected in the oviducts (1.01 and 0.24 ng/mg protein, respectively); however, neither peptide was detectable in myometrial tissue (less than 0.02 ng/mg protein). Our results demonstrate the presence of high concentrations of oxytocin and AVP in human ovarian and oviductal tissues as well as follicular fluid and suggest that neurohypophyseal peptides have a paracrine role in the regulation of ovarian or oviductal functions.

Effect of tamoxifen on stromal protein synthesis in the human prostate

To test the hypothesis that estrogen may have significant biological effects on the human prostate, we studied the effects of in vivo administration of tamoxifen, an antiestrogen, on human prostatic stromal protein synthesis in vitro. This was done by measuring [3H]proline incorporation in vitro in stromal cells separated enzymatically with 0.5% collagenase and trypsin from hypertrophic prostatic tissue removed at surgery. A mechanical method for separation of epithelium and stroma was attempted, but cell damage resulted in very low incorporation of [3H]proline into protein. Twelve patients were given 20 mg tamoxifen twice daily for 10 days before surgery. The serum levels of tamoxifen at the time of surgery ranged from 200-500 pmol/ml. Control tissues (14) were obtained from untreated patients. Tamoxifen decreased stromal protein synthesis, as measured by counts per min/mg protein, to approximately one third of that found in control patients (control mean, 102,428; tamoxifen-treated mean, 34,111; P less than 0.01). These results support the concept that estrogen has a significant role in the regulation of stromal protein synthesis and the pathophysiology of benign prostatic hypertrophy.

Accommodative disfacility presenting as intermittent exotropia

Accommodative disfacility and intermittent exotropia are two common clinically-observed ocular abnormalities. However, records of individual system dynamics are rare and mechanisms for possible interactive effects between the accommodation and the vergence system remain obscure in cases of system dysfunction. We present objective, quantitative, static and dynamic records of accommodation, vergence and related system responses in a symptomatic patient having accommodation disfacility and intermittent exotropia and propose that these two conditions worked in a pathogenic symbiotic manner.

Role of magnesium binding to myosin in controlling the state of cross-bridges in skeletal rabbit muscle

The effect of Mg2+ on the disposition of myosin cross-bridges was studied on myofibrils and synthetic myosin and rod filaments by employing chymotryptic digestion and chemical cross-linking methods. In the presence of low Mg2+ concentrations (0.1 mM), the proteolytic susceptibility at the heavy meromyosin/light meromyosin (HMM/LMM) junction in these three systems sharply increases over the pH range from 7.0 to 8.2. Such a change has been previously associated with the release of myosin cross-bridges from the filament surface [Ueno, H., & Harrington, W.F. (1981) J. Mol. Biol. 149, 619-640]. Millimolar concentrations of Mg2+ block or reverse this charge-dependent transition. Rod filaments show the same behavior as myosin filaments, indicating that the low-affinity binding sites for Mg2+ are located on the rod portion of myosin. The interpretation of these results in terms of Mg2+-mediated binding of cross-bridges to the filament backbone is supported by cross-linking experiments. The normalized rate of S-2 cross-linking in rod filaments at pH 8.0, kS-2/kLMM, increases upon addition of Mg2+ from 0.30 to 0.65 and approaches the cross-linking rate measured at pH 7.0 (0.75), when the cross-bridges are close to the filament surface. In rod filaments prepared from oxidized rod particles, chymotryptic digestion proceeds both at the S-2/LMM junction and at a new cleavage site located in the N-terminal portion of the molecule. Kinetic analysis of digestion rates at these two sites reveals that binding of Mg2+ to oxidized myosin rods has a similar effect at both sites over the pH range from 7.0 to 8.0.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuroendocrine control of the postpartum period

This postpartum period is characterized by stimulation of lactation and suppression of ovulation. During pregnancy a number of hormonal changes prepare the breasts for lactation following delivery. The process of lactation is functionally related to the suppression of ovulation. These two physiologic changes observed in the puerperium are considered in detail in this review.

An analysis of the direct effects of gossypol on human spermatozoa

The direct effects of gossypol and its acetic acid adduct, on the movement and functional competence of human spermatozoa were investigated employing exposure times of 1, 5 or 15 min and concentrations of 50 microM, 500 microM and 1000 microM. These compounds markedly reduced the motility, velocity, frequency of sperm head rotation and linearity of sperm progression, the most significant effects being observed with gossypol acetic acid on populations of 'capacitated' spermatozoa. Significant direct effects of gossypol on the ability of human spermatozoa to penetrate both cervical mucus and zona-free hamster ova were also observed, which were independent of any effects on motility. These results reinforce the notion that gossypol may serve a contraceptive role in the female as a 'spermicidal' agent, and suggest that this compound may also be of scientific value as a probe for identifying and isolating functionally important components of the human spermatozoon.

The influence of medium composition, osmolarity and albumin content on the acrosome reaction and fertilizing capacity of human spermatozoa: development of an improved zona-free hamster egg penetration test

The influence of medium composition, osmolarity and albumin concentration on the ability of human spermatozoa to undergo the acrosome reaction and penetrate zona-free hamster ova has been investigated. Raising the osmolarity but not the albumin concentration of the media was found to significantly increase the proportion of spermatozoa exhibiting an acrosome reaction and penetrating hamster ova, without influencing motility. There was, however, no correlation between the size of the acrosome reacted population and penetration rates between samples, suggesting that the zona-free hamster egg penetration test is more than just a measure of the availability of acrosome reacted cells. As a result of this study, a revised protocol for the hamster egg assay is described which is shorter and considerably more sensitive than conventional procedures.

Premonitory symptoms of stroke in evolution to the locked-in state

Three patients, who subsequently developed the locked-in state characterised by quadriplegia and mutism with an alert sensorium, initially had mild dysarthria and uncrossed hemisensory or hemimotor deficits involving the face and ipsilateral extremities. Case one initially mimicked a left cerebral lesion with right hemisensory deficits, a mild right facial paresis and a right homonymous field deficit. Case two initially developed both left hemimotor and hemisensory deficits and later developed a paresis of right conjugate gaze. Case three presented with left hemimotor deficit, and mild paresis of conjugate gaze to the right. All three patients died. Rostral brainstem infarctions were found at necropsy in cases one and two. Case three had a radiolucent area of the brainstem demonstrated by CT Scan. Hemisensory and hemimotor deficits also have been noted to precede reported cases of pontine infarction with the locked-in state. Acute onset of uncrossed hemisensory and hemimotor deficits with dysarthria may be caused by infarction of the pons which may predispose to the locked-in state.

The application of enzyme immunoassay to the study of salivary IGA

In the study of salivary IgA, solid-phase enzyme immunoassay has been able to show that 1) the assay is reliable, 2) minimal loss of IgA occurs from centrifuging or by elimination of insoluble mucins, 3) IgA can be easily measured in fractions from sucrose gradient sedimentation, 4) freezing alone is sufficient to preserve salivary IgA, 5) the concentration in normal saliva in 11 persons ranged from 15 to 240 mg/L (-/x = 81), and 6) an abnormal saliva contained less than 6 ug/L, the detection limit for this EIA.

Purification and partial characterization of hyaluronidase from five pace snake (Agkistrodon acutus) venom

A hyaluronidase (EC 3.2.1. 35) was isolated and purified from Agkistrodon acutus venom. The purification procedure included CM-Sephadex C-50 chromatography, gel-filtration on Sephadex G-75 and CM-Sephadex C-25 chromatography. The purified preparation of the enzyme was homogeneous on polyacrylamide gel electrophoresis at pH 4.3, a 45-fold purification being obtained. The hyaluronidase was a glycoprotein (positive PAS staining) with a molecular weight of 33,000 and a pI of 10.3. No hemorrhagic activity was found. The hyaluronidase had an optimum pH of 3.5-5.0 and an optimum temperature of 37 degrees C. It was heat sensitive, was more stable in the acidic than in the neutral region, and lost its activity in the alkaline region. Fe2+, Cu2+ and heparin inhibited the venom hyaluronidase. The Km value for hyaluronic acid was 6.2 X 10(-3) mg/ml.

The action of six antagonists of the excitatory amino acids on neurones of the rat spinal cord

The differential susceptibility to blockade by six antagonists of the excitatory actions produced by a number of acidic amino acids has been determined. Four of these, DL-alpha-aminoadipate, 2-amino-5-phosphonovalerate, 2-amino-6-phosphonocaproate and gamma-D-glutamylglycine block the action of N-methyl-D-aspartate (NMDA) most effectively and those of L-glutamate, trans-1-amino-1,3-dicarboxycyclopentane (trans-ADCP) and kainate the least. Cis-2,3-piperidine dicarboxylate resembles the other four, save in that kainate excitations are also powerfully blocked. L-glutamate diethylester was most effective against L-glutamate and least against NMDA: kainate and trans-ADCP excitations were also relatively unaffected. The data support the hypothesis of the existence of at least three classes of amino acid receptor (NMDA-, L-glutamate- and kainate-excited), and the possibility of a fourth category activated by trans-ADCP must be considered.

Cerebral edema and ophthalmoplegia reversed by mannitol in a new case of insulin-dependent diabetes mellitus

Cerebral edema is a sometimes fatal complication of diabetic ketoacidosis which occurs unpredictably and when biochemical parameters show improvement. A case of a young, newly diagnosed insulin-dependent diabetic boy who developed this complication while receiving a low-dose continuous insulin infusion is reported. Two hours after treatment signs of headache, ophthalmoplegia, and blurred disc margins suggested early cerebral edema. Despite fluid restriction, avoidance of alkali, and phosphate supplementation, cerebral edema ensued three hours later. This complication was then reversed by administration of mannitol. Our patient's ophthalmoplegia, unlike typical diabetic ophthalmoplegia, improved immediately and completely resolved within two weeks after this episode. It is concluded that the use of mannitol in the cerebral edema of diabetic ketoacidosis is beneficial if it is instituted promptly.

Interaction of myosin subfragment 1 with Cibacron Blue F3GA

Cibacron Blue F3GA and its immobilized derivatives have been shown before to bind and inhibit nucleotide-dependent enzymes and, among them, myosin subfragment 1. Experiments have been carried out to examine the mechanism of the subfragment 1--dye interaction. Binding of subfragment 1 to immobilized dye (Affi-Gel Blue) does not involve the ATP binding site on myosin. Subfragment 1 hydrolyzes MgATP and CaATP while bound to the Affi-Gel Blue column. Inactivated subfragment 1, which contains [3H]ADP noncovalently trapped at the active site, binds and elutes from the Affi-Gel Blue column in the same manner as unmodified, active protein. Free Cibacron Blue inhibits the ATPase activity of subfragment 1. The inhibition is pH, salt, and time dependent. Complete inhibition correlates with the noncovalent binding of four to five dye molecules per mole of subfragment 1. Three to four of these dye molecules can be preferentially removed from subfragment 1 in the presence of 1 M KCl without relieving the inhibition. This inhibition, which can be traced to one dye molecule per subfragment 1, is reversible and is facilitated in the presence of MgADP and MgATP, suggesting that the dye does not bind at the active site of subfragment 1. Our observations are explained in terms of hydrophobic and electrostatic protein--dye interactions.

Solid-phase selection of human T lymphocyte subpopulations using monoclonal antibodies

This report describes a novel solid-phase technique for the positive selection of human T lymphocyte subsets labeled by indirect immunofluorescence with monoclonal anti-bodies. Fluorescein labeled normal human T cells or a T cell line were fractionated on plastic culture dishes precoated with affinity chromatography purified anti-fluorescein antibodies. Cell binding was specific for fluorescein, and was both time and temperature dependent. Bound cells were eluted at 37 degrees C with fluorescein-L-lysine. The eluted cells were enriched with highly viable and functional human T cell subsets. Thus Leu3 monoclonal antibody selected cells were shown to provide helper activity in the pokeweed mitogen induced IgM and IgG immunoglobulin secretory response of autologous B cells. The Leu2 antibody selected T cells suppressed both IgM and IgG secretory responses. In addition, studies with the monoclonal antibody 1G11, which binds to an antigen expressed on acute lymphocytic leukemia (T-ALL) cells and the T-ALL derived cell line RPMI-8402, demonstrated that this solid-phase technique can be used to select for cells which are present at low frequencies in a mixed population. It thus provides a simple and reproducible means for the preparative isolation of lymphocyte subsets associated with autoimmune and neoplastic disease for functional and biochemical analysis.

Comparative pharmacokinetics of 1-(2-chloroethyl)-3-(2,6-dioxo-1-piperidyl)-1-nitrosourea in rats and patients and extrapolation to clinical trials

The plasma pharmacokinetics of 1-(2-chloroethyl)-3-(2,6-dioxo-1-piperidyl)1-nitrosourea (PCNU) were determined in ambulatory rats and in patients receiving PCNU chemotherapy in Phase 1 and II studies. After derivativization to the methyl carbamate, both rat and human PCNU plasma levels were measured by gas chromatography-mass spectrometry. Comparison of the tolerated dose levels and pharmacokinetics of PCNU to the values determined for 1,3-bis(2-chloroethyl)-1-nitrosourea in humans indicated that PCNU has a lower plasma drug area under the curve at equitoxic doses. We conclude that PCNU may show less clinical efficacy than 1,3-bis(2-chloroethyl)-1-nitrosourea in the treatment of solid tumors.

Lymphocytotoxic anti-LewisbH antibody

We wish to report strong lymphocyte crossmatch incompatibility attributable to anti-Lea antibody. The prospective renal transplant recipient was Le(a-b-) and her serum contained potent anti-Lea and anti-LebH antibodies. Despite the fact that were able to demonstrate greater B lymphocyte than T lymphocyte Lewis(a+) antigenicity, this serum was capable of causing greater than 80% cytotoxicity of Le(a+) and Le(b+)O or A2 whole lymphocyte populations. Antibody activity was completely inhibited by Lewis substance. This inhibition was specific and did not interfere with HLA antibody activity.

The postexcitatory effects of acidic amino acids on spinal neurones

The excitation of some neurones in the spinal cord of rats by L-glutamate and L-aspartate is followed by a period of reduced excitability. This effect is not observed after excitation by D-glutamate nor, in the case of Renshaw cells, by acetylcholine. The depressions following L-glutamate were reduced by bicuculline and those after aspartate by strychnine, suggesting that they may have been caused through decarboxylation of the excitatory amino acids to yield the inhibitory compounds gamma-aminobutyric acid and beta-alanine, respectively.

The interaction of Cibacron Blue F3GA with troponin and its subunits

Binding of troponin to Cibacron Blue F3GA-agarose column and its selective release from the gel in the presence of 0.5 M KCl provides the basis for a new purification method. The two-step procedure consists of isoelectric precipitation of tropomyosin and chromatography of the resultant crude troponin supernatant on Affi-Gel Blue column. Adsorption of troponin to the immobilized dye appears to occur through the troponin-T subunit. Troponin-I and troponin-C do not bind to the blue agarose column, whereas troponin-T binds to it very tightly. Binding of the dye to troponin-T prevents formation of troponin-T-troponin-C complex, but does not interfere with direct interaction of troponin-T with troponin-I. The activity of troponin in conferring calcium sensitivity on actomyosin ATPase is not affected by Cibacron Blue. Circular dichroism and difference absorption measurements of complexes of the blue dye with troponin and its subunits reveal the presence of a tight binding site on whole troponin and on troponin-T (KA greater than or equal to 10(6) M). The existence of weak binding sites for the dye on troponin and all of its subunits is deduced from difference absorption studies. Cibacron Blue appears to be a sensitive probe for subunit interactions in troponin.

Dissimilar effects of ultraviolet light on HLA-D and HLA-DR antigens

Since HLA-DR antigens are closely related to HLA-D antigens, we studied the effect of low-dose ultraviolet irradiation on the ability of human lymphocytes to induce a proliferative response in mixed lymphocyte culture (HLA-D) and on the serologic reactivity of B lymphocyte HLA-DR antigens. We found (1) no alteration in the serologic definition of DR antigens despite the abrogation of the induction of proliferative responses in mixed lymphocyte culture, (2) no evidence that ultraviolet light (UVL) stimulated suppressor cells contributed to the loss of allostimulation, and (3) no evidence that the abrogation of the induction of proliferative responses could be modified by increasing the number of UVL exposed lymphocytes in mixed lymphocyte culture. These findings suggest that HLA-D and HLA-DR antigens are different molecularly, but are also consistent with the concept that certain active metabolic processes are required of stimulator cells for allostimulation and that these processes are UVL sensitive.

The frequency of suicides around major public holidays: some surprising findings

This paper analyzes the recorded number of U.S. daily suicides, 1972-1976. The paper shows that: there is a dip in suicides on and just before major public holidays; there is a peak in suicides just after these holidays; and, the dip in suicides is somewhat larger than the peak--hence, there is a net decrease in suicides around the holidays. On balance, the holidays appear to reduce the number of suicides, not increase them, as previously believed. In addition, previous research has focussed on the negative psychological impact of holidays. Our findings indicate that holidays have a positive component, which should be taken into account in future research.

Radiation exposure to medical personnel during iodine-125 seed implantation of the prostate

Radiation exposure to operating room personnel was measured by thermoluminescent dosimeters and an ionization chamber during a simulated iodine-125 seed implantation of the prostate gland. The exposure rates at points around the implanted phantom were measured and compared with the calculated values. The data suggested that even with a schedule of 25 prostate implants per year, exposure levels would not exceed 5% of the recommended maximum limits for occupationally exposed personnel.

Accommodation dynamics I. Range nonlinearity

We show the presence of a new nonlinearity in accommodation dynamics. Positive accommodation is slower when the eye is operating in the near range; conversely, relaxation of accommodation is apparently slower during work in the far range. Latency of accommodation is unaffected by working range. This new dynamic range nonlinearity has been defined in connection with a continuing research study of accommodation responses in normal subjects and in clinic patients. Instrumentation and analysis procedures are described.

Effect of hyperoxia, hypoxia, and maturation on superoxide dismutase activity in isolated alveolar macrophages

The influence of ambient O2 tensions and of cell maturation on superoxide dismutase activity were studied in tissue culture--maintained mouse alveolar macrophages. Cultivation under hyperoxic conditions (PO2 about 640 mmHg) for 24 hours was associated with a significant increase in superoxide dismutase activity as compared with normoxic conditions (PO2 approximately 150 mmHg). (Hyperoxia: superoxide dismutase = 7.9 +/- 4.0 (SD); normoxia: superoxide dismutase = 4.4 +/- 1.7 units X mg cell protein-1 P less than 0.05). Hypoxic exposure (PO2 approximately 15 mmHg) was associated with a significant decrease in superoxide dismutase compared to normoxic controls (hypoxia: 2.2 +/- 0.6; normoxic: 3.8 +/- 0.6 units X mg protein-1 P less than 0.01). This decrease was found only after 168 hours of in vitro hypoxia. The in vitro maturation of alveolar macrophages cultivated in air was associated with a progressive increase in superoxide dismutase activity per 10(6) cells, although superoxide dismutase activity per unit protein remained constant. Molecular O2 may modify cell superoxide dismutase activity by altering intrinsic enzyme regulation. The increase in superoxide dismutase activity with hyperoxia and the decrease with hypoxia are consistent with but not unequivocally establish an important role for superoxide dismutase in protecting against cellular O2 toxicity.