Simulation of the electroretinographic slow PIII component by injection of K+ -free Ringer's solution in the frog retina

The light-induced hyperpolarization of the photoreceptor results in a decrease in extracellular potassium ion concentration, [K+]. A decrease in [K+] in the photoreceptor layer was caused by injecting a small amount of K+-free Ringer's solution in the isolated frog retina, and the distribution of the field potential as induced by K+-free Ringer's solution was mapped. The depth profile of the response resembled that of the slow PIII component of the electroretinogram. These results support the notion that the slow PIII is generated by the Müller cells as they hyperpolarize in response to a light-induced decrease in [K+] in the photoreceptor layer.

Inhibition of sliding movement of F-actin by crosslinking emphasizes the role of actin structure in the mechanism of motility

The effects of crosslinking of monomeric and polymeric actin with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC), disuccinimidyl suberate (DSS) and glutaraldehyde on the interaction with heavy meromyosin (HMM) in solution and on the sliding movement on glass-attached HMM were examined. The Vmax values of actin-activated HMM ATPase decreased in the following order: intact actin = EDC F-actin greater than DSS actin greater than glutaraldehyde F-actin = glutaraldehyde G-actin greater than EDC G-actin. The affinity of actin for HMM in the presence of ATP decreased in the following order: DSS actin greater than glutaraldehyde F-actin = glutaraldehyde G-actin greater than intact actin greater than EDC F-actin greater than EDC G-actin. However, sliding movement was inhibited only in the case of glutaraldehyde-crosslinked F and G-actin and EDC-crosslinked G-actin. Interestingly, after copolymerization of "non-motile" glutaraldehyde or EDC-crosslinked monomers with "motile" monomers of intact actin sliding of the copolymers was observed and its rate was independent of the type of crosslinked monomer, i.e. of the manner of their interaction with HMM. These data strongly indicate that inhibition of the sliding of actin by crosslinking cannot be explained entirely by changes in the Vmax value or affinity for myosin heads. We conclude that movement is generated by interaction of myosin with segments of F-actin containing a number of intact monomers, and the mechanism of inhibition involves an effect of the crosslinkers on the structure of F-actin itself.

Mechanochemical coupling in actomyosin energy transduction studied by in vitro movement assay

In order to study the mechanochemical coupling in actomyosin energy transduction, the sliding distance of an actin filament induced by one ATP hydrolysis cycle was obtained by using an in vitro movement assay that permitted quantitative and simultaneous measurements of (1) the movements of single fluorescently labeled actin filaments on myosin bound to coverslip surfaces and (2) the ATPase rates. The sliding distance was determined as (the working stroke time in one ATPase cycle, tws) x (the filament velocity, v). tws was obtained from the ATPase turnover rate of myosin during the sliding (kt), the ATP hydrolysis time (delta t) and the ON-rate at which myosin heads enter into the working stroke state when they encounter actin (kON); tws approximately 1/kt-delta t-1/kON. kt was estimated from the ATPase rates of the myosin-coated surface during the sliding of actin filaments. delta t has been determined as less than 1/100 per second, kON was estimated by analyzing the movements of very short (40 nm) filaments. The resulting sliding distance during one ATP hydrolysis cycle near zero load was greater than 100 nm, which is about ten times longer than that expected for a single attachment-detachment cycle between an actin and a myosin head. This leads to the conclusion that the coupling between the ATPase and attachment-detachment cycles is not determined rigidly in a one-to-one fashion.

Loose coupling between chemical and mechanical reactions in actomyosin energy transduction

In order to study the mechanochemical coupling in actomyosin energy transduction, the sliding distance of an actin filament induced by a myosin head during one ATP hydrolysis cycle was obtained using an in vitro movement assay, which permitted quantitative and simultaneous measurements of (1) the movements of single fluorescently-labeled actin filaments on myosin bound to coverslip surfaces and (2) the ATPase rates. The sliding distance was determined as (the working-stroke time in one ATPase cycle, Tws) x (the filament velocity, v). The working-stroke time (Tws) was obtained from the ATPase turnover rate of myosin during the sliding (kT), the ATP hydrolysis time on myosin heads (delta T) and the ON-rate at which myosin heads enter into the working-stroke when they encounter actin (kON); Tws approximately 1/kT-delta T-1/kON. kON was estimated by analyzing the movements of very short (40 nm) filaments. The resulting sliding distance during one ATP hydrolysis cycle near zero load was greater than 100 nm, which is about 10 times longer than that expected for a single attachment-detachment cycle between an actin monomer and a myosin head. This leads to the conclusion that the coupling between the chemical and mechanical reactions is not rigid in a one to one fashion.

Kinetics of HDL-apo A-I in the WHHL rabbit, an animal model of familial hypercholesterolemia

Plasma high density lipoproteins (HDL) and their major protein, apolipoprotein (apo) A-I, are deficient in homozygous Watanabe heritable hyperlipidemic (WHHL) rabbits. As the pathophysiology of subnormal HDL-apo A-I is unclear, we examined kinetic parameters of HDL-apo A-I in WHHL rabbits and normolipidemic Japanese White (control) rabbits. The total plasma mass of HDL-apo A-I was significantly smaller (P less than 0.01) in WHHL rabbits than in controls (less than one-third). Mean fractional catabolic rates (FCR) computed from the specific radioactivity decay curves of HDL-apo A-I were significantly greater (P less than 0.01) in WHHL rabbits than in controls (0.874 +/- 0.052 vs. 0.502 +/- 0.060/day, respectively). Mean synthetic rates of HDL-apo A-I were significantly lower (P less than 0.01) in WHHL rabbits than in controls (8.67 +/- 0.59 vs. 18.14 +/- 3.75 mg/kg body weight/day, respectively). We conclude that the deficiency of plasma HDL-apo A-I in the WHHL rabbits resulted from an increase in FCR and a decrease in apo A-I synthesis.

[Simulation of the electroretinographic slow P III component by injection of K+-free Ringer's solution in the frog retina]

A decrease in extracellular potassium concentration [( K+]o) was caused by injecting a small amount of K+ -free Ringer's solution into the photoreceptor layer of the isolated frog retina. The response induced by K+ -free Ringer's solution was receptor side-positive, and the depth profile of the response resembled that of the response (slow P III) induced by spot illumination. This result supports the notion that the slow P III is generated by Müller cells as they hyperpolarize in response to a light-induced decrease in [K+]o in the photoreceptor layer.

Construction of a secretion vector production of peptide hormones in Escherichia coli (extracellular production of calcitonin as fused protein)

A new secretion vector, pEAP84 which contained a unique restriction site (BglII) at the 3' end of the penicillinase gene to produce a fused protein, and the Ex-kil region to make the outer membrane permeable, was constructed from pEAP82. A recombinant plasmid p84h06, which contained a synthetic gene for human calcitonin with a cyanogen bromide cleavage site at the junction site of the fused protein, was constructed and introduced into Escherichia coli. The hybrid protein produced in E. coli carrying p84h06 was secreted into the culture medium. The amino acid composition of this product was consistent with that deduced from the DNA sequence. Mature calcitonin was obtained following cyanogen bromide cleavage of the fused protein.

Force measurements by micromanipulation of a single actin filament by glass needles

Single actin filaments (approximately 7 nm in diameter) labelled with fluorescent phalloidin can be clearly seen by video-fluorescence microscopy. This technique has been used to observe motions of single filaments in solution and in several in vitro movement assays. In a further development of the technique, we report here a method to catch and manipulate a single actin filament (F-actin) by glass microneedles under conditions in which external force on the filament can be applied and measured. Using this method, we directly measured the tensile strength of a filament (the force necessary to break the bond between two actin monomers) and the force required for a filament to be moved by myosin or its proteolytic fragment bound to a glass surface in the presence of ATP. The first result shows that the tensile strength of the F-actin-phalloidin complex is comparable with the average force exerted on a single thin filament in muscle fibres during isometric contraction. This force is increased only slightly by tropomyosin. The second measurement shows that the myosin head (subfragment-1) can produce the same ATP-dependent force as intact myosin. The magnitude of this force is comparable with that produced by each head of myosin in muscle during isometric contraction.

Direct observation of molecular motility by light microscopy

We used video-fluorescence microscopy to directly observe the sliding movement of single fluorescently labeled actin filaments along myosin fixed on a glass surface. Single actin filaments labeled with phalloidin-tetramethyl-rhodamine, which stabilizes the filament structure of actin, could be seen very clearly and continuously for at least 60 min in 02-free solution, and the sensitivity was high enough to see very short actin filaments less than 40 nm long that contained less than eight dye molecules. The actin filaments were observed to move along double-headed and, similarly, single-headed myosin filaments on which the density of the heads varied widely in the presence of ATP, showing that the cooperative interaction between the two heads of the myosin molecule is not essential to produce the sliding movement. The velocity of actin filament independent of filament length (greater than 1 micron) was almost unchanged until the density of myosin heads along the thick filament was decreased from six heads/14.3 nm to 1 head/34 nm. This result suggests that five to ten heads are sufficient to support the maximum sliding velocity of actin filaments (5 micron/s) under unloaded conditions. In order for five to ten myosin heads to achieve the observed maximum velocity, the sliding distance of actin filaments during one ATP cycle must be more than 60 nm.

Microelectrode depth study of the electroretinographic oscillatory potentials in the frog retina

The depth profile of the electroretinographic oscillatory potentials was studied in the isolated frog retina. The intraretinal electrode was introduced from the receptor side, and the reference electrode was placed on the vitreal side. The electroretinogram, recorded either transretinally or with the electrode tip in the receptor layer, showed 4 to 10 oscillatory potential wavelets. As the electrode was advanced proximally, the wavelets disappeared as a function of retinal depth. The wavelets with longer peak latencies disappeared earlier, and only the first one or two wavelets could be identified when the electrode was in the inner plexiform layer. These findings indicate that the oscillatory potentials are generated between the inner and outer plexiform layers and that the earlier wavelets originate in the more proximal retina. The results are consistent with the notion that the oscillatory potentials are generated by synaptic feedback circuits.

Sliding movement of single actin filaments on one-headed myosin filaments

The myosin molecule consists of two heads, each of which contains an enzymatic active site and an actin-binding site. The fundamental problem of whether the two heads function independently or cooperatively during muscle contraction has been studied by methods using an actomyosin thread, superprecipitation and chemical modification of muscle fibres. No clear conclusion has yet been reached. We have approached this question using an assay system in which sliding movements of fluorescently labelled single actin filaments along myosin filaments can be observed directly. Here, we report direct measurement of the sliding of single actin filaments along one-headed myosin filaments in which the density of heads was varied over a wide range. Our results show that cooperative interaction between the two heads of myosin is not essential for inducing the sliding movement of actin filaments.

Suppression of the hyperosmolarity response after cataract surgery

The hyperosmolarity response was recorded with the electrooculogram in the dark before and after cataract extraction, to investigate a possible disorder in the retinal pigment epithelium after cataract extraction. The hyperosmolarity response was suppressed one week after surgery, especially after intracapsular extraction. The response recovered up to the preoperative level in most cases one or two months after surgery. These findings indicate reversible functional disorders of the pigment epithelium after cataract extraction, particularly intracapsular. This dysfunction can be revealed by the hyperosmolarity response. This response is clinically useful for diagnosis of pigment epitheliopathy after cataract extraction.

Roles of lysine1 and lysine7 residues of bovine pancreatic ribonuclease in the enzymatic activity

In order to investigate the roles of Lys1 and Lys7 of RNase A in the enzymatic activity, four S-peptide derivatives were prepared and their abilities to activate S-protein were measured. They are 1-norleucine-S-peptide, 7-norleucine S-peptide, 1,7-di-norleucine-S-peptide, and tri-N-acetyl S-peptide. From the analyses of the relative activity and kinetic parameters of RNase S' derivatives with UpU, UpU greater than p, and UpUpU greater than p, it was concluded that Lys7 of RNase A is a binding site for 3'-phosphate of UpU greater than p and the modification or substitution of Lys1 affects the binding of trinucleotide substrate.

Sliding distance of actin filament induced by a myosin crossbridge during one ATP hydrolysis cycle

Muscle contraction results from a sliding movement of actin filaments induced by myosin crossbridges on hydrolysis of ATP, and many non-muscle cells are thought to move using a similar mechanism. The molecular mechanism of muscle contraction, however, is not completely understood. One of the major problems is the mechanochemical coupling at high velocity under near-zero load. Here, we report measurements of the sliding distance of an actin filament induced by a myosin crossbridge during one ATP hydrolysis cycle in an unloaded condition. We used single sarcomeres from which the Z-lines, structures which anchor the thin filaments in the sarcomere, had been completely removed by calcium-activated neutral protease (CANP) and trypsin, and measured both the sliding velocity of single actin filaments along myosin filaments and the ATPase activity during sliding. Our results show that the average sliding distance of the actin filament is less than or equal to 600 A during one ATP cycle, much longer than the length of power stroke of myosin crossbridges deduced from mechanical studies of muscle, which is of the order of 80 A (for example, ref. 15).

Angle of active site of myosin heads in contracting muscle during sudden length changes

The change in orientation of myosin crossbridges in contracting muscle during sudden length changes was examined by fluorescence polarization. This study used a fluorescent ATP analogue, 1,N6-etheno-2-aza-ATP(epsilon-2-aza-ATP) as a probe. Its fluorescence is considerably enhanced upon binding with myosin and is dependent on the chemical state of the myosin-nucleotide complex in muscle. The results showed that nucleotides bound to crossbridges in the intermediate attached state (presumably AM-epsilon-2-aza-ADP-Pi) during isometric contraction are highly oriented at the same angle as that of AM in rigor with bound epsilon-2-aza-ADP. Furthermore the orientation of nucleotides bound to crossbridges in the attached state is not altered during sudden changes in length of isometrically contracting muscle. The results of this time-resolved measurement support the conclusion obtained from a previous steady-state experiment that change in axial orientation of the active site of the myosin head is not involved in force generation.

Predominant attached state of myosin cross-bridges during contraction and relaxation at low ionic strength

The chemical states of a cross-bridge--nucleotide complex were studied using a fluorescent ATP analogue, 1-N6-etheno-2-aza-ATP(epsilon-2-aza-ATP). The fluorescence of epsilon-2-aza-ATP at specific emission wavelengths was enhanced by 12.5 times upon binding to myosin in a relaxed muscle and the fluorescence from the resultant myosin(M)-epsilon-2-aza-ADP-Pi intermediate was 2.5 times greater than that from a M-epsilon-2-aza-ADP complex. Similar enhancements of the fluorescence of epsilon-2-aza-ATP and epsilon-2-aza-ADP were observed upon binding to heavy meromyosin in solution. Binding of F-actin did not change the fluorescence of epsilon-2-aza-ATP or epsilon-2-aza-ADP bound to heavy meromyosin. When a muscle went from a relaxed state to a state of isometric contraction or contraction with shortening, the fluorescence intensity decreased only slightly or not at all, i.e. the fluorescence of nucleotides bound to most of the myosin heads during contraction is the same as that of the M-epsilon-2-aza-ADP-Pi intermediate. These results suggest that an actomyosin(AM)-epsilon-2-aza-ADP-Pi intermediate is the predominant attached state during contraction. When the ionic strength of the relaxing solution was decreased, cross-bridges formed at 6 degrees C without tension generation. At 20 degrees C, a large tension was produced although the shortening velocity was negligibly small or zero. The fluorescence intensity decreased by 15% at 20 degrees C but only a small decrease of 3% was observed at 6 degrees C, suggesting that the predominant complexes in the attached state were AM-epsilon-2-aza-ATP and/or AM-2-aza-ADP-Pi at 6 degrees C and AM-epsilon-2-aza-ADP at 20 degrees C. Thus, the identification of the actomyosin-nucleotide complexes existing before and after the force-generating step lent further support to the conclusion that the sliding force is generated by conformational changes in actomyosin when the (epsilon-2-aza-)ADP-Pi complex is bound to it.

[The clinical significance of tissue polypeptide antigen (TPA) in the patients with gynecologic tumor]

In order to estimate the clinical significance of tissue polypeptide antigen (TPA), TPA was measured by radioimmunoassay in sera from patients with various gynecological tumors. They were 40 uterine myomas, 94 cervical cancers, 21 endometrial cancers, 3 vulval cancers, 51 benign ovarian tumors and 78 malignant ovarian tumors including 18 low potential malignant tumors (LPM). The mean TPA values in patients with benign as well as malignant tumors were significantly higher than that of 97 healthy volunteers (68 +/- 17 U/l; Upper limit; 107 U/l). Among the cervical cancer patients, serum TPA level and positive ratio became higher as the disease progressed. In the advanced cases, the mean serum TPA value and positive ratio were 149 +/- 64 U/l and 75%, respectively. The mean TPA value in the endometrial cancer patients was significantly higher than that of myoma patients. Among the patients with ovarian tumor, serum TPA was elevated in 14% of benign cases, 28% of LPM cases, 47% of stage I cases and 82% of the advanced cases. Serum TPA values varied directly with the stage and malignancy of disease. The present study revealed that TPA is a useful markers in the diagnosis of gynecological tumors, especially for ovarian cancers.

Direct observation of motion of single F-actin filaments in the presence of myosin

Actin is found in almost all kinds of non-muscle cells where it is thought to have an important role in cell motility. A proper understanding of that role will only be possible when reliable in vitro systems are available for investigating the interaction of cellular actin and myosin. A start has been made on several systems, most recently by Sheetz and Spudich who demonstrated unidirectional movement of HMM-coated beads along F-actin cables on arrays of chloroplasts exposed by dissection of a Nitella cell. As an alternative approach, we report here the direct observation by fluorescence microscopy of the movements of single F-actin filaments interacting with soluble myosin fragments energized by Mg2+-ATP.

Angles of fluorescently labelled myosin heads and actin monomers in contracting and rigor stained muscle fiber

The measurement of polarized fluorescence from a fluorescent ATP or ADP analog, epsilon-ATP or epsilon-ADP (1, N6-Etheno-ATP or ADP) bound to myosin heads in a glycerinated muscle fiber revealed that during isometric contraction at low concentrations of Mg-epsilon-ATP the bound nucleotides are highly oriented with respect to the fiber axis, and their mean angle is almost the same as that in rigor. Furthermore, the polarization of tryptophan fluorescence did not change when the fiber was transferred from a rigor state to an active state at low ATP concentrations. Thus, if a rotation of myosin heads occurs during contraction, it seems to be very limited. The angle of bound epsilon-ADP is not changed by passive stretching of the fiber in the presence of epsilon-ADP. By using the same technique, it was found that orientation of a phalloidin-FITC complex (Ph-FITC) specifically bound to F-actin in a glycerinated muscle fiber is significantly changed when the fiber is activated from relaxation or rigor to contraction. No change in orientation of Ph-FITC bound to F-actin is induced by simple addition of Mg-ADP or by passive stretching of the fiber in the absence of ATP. These results suggest that a rotation or a distortion of actin monomers occurs during contraction, which is involved in the process of active tension development.

Studies on conformation of F-actin in muscle fibers in the relaxed state, rigor, and during contraction using fluorescent phalloidin

F-actin in a glycerinated muscle fiber was specifically labeled with fluorescent phalloidin-(fluorescein isothiocyanate) FITC complex at 1:1 molar ratio. Binding of phalloidin-FITC to F-actin affected neither contraction of the fiber nor its regulation by Ca2+. Comparison of polarized fluorescence from phalloidin-FITC bound to F-actin in the relaxed state, rigor, and during isometric contraction of the fiber revealed that the changes in polarization accompanying activation are quantitatively as well as qualitatively different from those accompanying transition of the fiber from the relaxed state to rigor. The extent of the changes of polarized fluorescence during isometric contraction increased with decreasing ionic strength, in parallel with increase in isometric tension. On the other hand, polarized fluorescence was not affected by addition of ADP or by stretching of the fiber in rigor solution. It is concluded from these observations that conformational changes in F-actin are involved in the process of active tension development.

[Moderate dose methotrexate with citrovorum factor rescue therapy in the treatment of head and neck cancer]

A clinical trial of moderate dose methotrexate (MTX)-CF rescue was conducted in 12 institutions. Thirty-seven patients with head and neck carcinoma entered this trial, of which 32 were evaluable. MTX was administered 350 mg/m2 (500 mg/body) by i.v. drip over 6 hours. Three hours after completion of MTX infusion, CF rescue was started. There was no complete response in 32 patients. Nine patients showed partial response with the response rate of 28%. The response rates were 21% for the group of patients treated previously, and 75% for the group untreated previously. MTX concentration in plasma was determined at 6, 24, 48 and 72 hours after the initiation of MTX infusion, and the assay results revealed a safe range. GI disturbances were seen at the rates of 11 to 38%. Bone marrow suppression was mild and no renal toxicity was observed. We concluded that the moderate dose MTX-CF rescue therapy was useful for head and neck carcinoma. As a next step, we are planning to conduct a clinical trial of high-dose MTX.

The effect of crosslinking of thin filament with glutaraldehyde on the contractility of muscle fiber

The effect of crosslinking of F-actin with glutaraldehyde on the contractility of muscle ghost fiber containing reconstituted thin filament (i.e. F-actin-tropomyosin-troponin complex) and irrigated with myosin was investigated. The results show that: (i) crosslinking inhibited development of isometric tension and shortening of the fiber in the presence of MgATP, (ii) superprecipitation of the complex of myosin with crosslinked thin filament was considerably delayed, (iii) crosslinking inhibited neither binding of myosin heads to the filament nor activation of myosin ATPase. It is suggested that alterations of actin structure due to the formation of intra- and/or intermonomer crosslinks can essentially affect the process of contractility.

Interaction of myosin with thin filaments during contraction and relaxation: effect of ionic strength

The influence of ionic strength on the isometric tension, stiffness, shortening velocity and ATPase activity of glycerol-treated rabbit psoas muscle fiber in the presence and the absence of Ca2+ has been studied. When the ionic strength of an activating solution (containing Mg2+-ATP and Ca2+) was decreased by varying the KCl concentration from 120 to 5 mM at 20 degrees C, the isometric tension and stiffness increased by 30% and 50%, respectively. The ATPase activity increased 3-fold, while the shortening velocity decreased to one-fourth. At 6 degrees C, similar results were obtained. These results suggest that at low ionic strengths ATP is hydrolyzed predominantly without dissociation of myosin cross-bridges from F-actin. In the absence of Ca2+, with decreasing KCl concentration the isometric tension and stiffness developed remarkably at 20 degrees C. However, the ATPase activity and shortening velocity were very low. At low ionic strength, even in the absence of Ca2+ myosin heads are bound to thin filaments. The development of the tension and stiffness were greatly reduced at 6 degrees C or at physiological ionic strength.

Local potassium concentration changes in the retina and the electroretinographic (ERG) b-wave

Intraretinal injections of high- and low-K+ Ringer's solutions generate potentials which are opposite in polarity but otherwise similar in both their shapes and retinal depth profiles. We provide evidence that the principal generator of the K+-mediated potential is the Müller (glial) cell, but that the FRG b-wave cannot be explained solely by this K+-mediated potential without considering involvement of the electrical activities of neuronal cells.

[Pharmacokinetics of cis-platinum diammine dichloride]

Levels of cis-platinum diammine dichloride (CPDD) were determined in the serum, urine and ascites by the flameless atomic absorption spectrophotometry. Samples were obtained from the patients with ovarian cancer (5 cases) and cervical cancer (1 case). The patients were given CPDD by infusion every three weeks. The treatment schedules were as follows: A, 14-16 mg/body/day X 5 days; B, 20 mg/body/day X 5 days; C, 50 mg/body X 1 day. The levels of CPDD in the serum of the patients treated under schedules A and B became higher after the repeated administration from day 1 through 5. In the patients treated under schedules A and B, a more complex serum elimination curve was seen, while a simple biphasic clearance pattern was observed in the patient treated under schedule C. The concentration of CPDD in the serum arose with the increase of doses. A measurable concentration of CPDD was still detected 14 days after the end of administration. The urinary recovery at 24 hours after administration of CPDD ranged from 17.0 to 21.0% under schedules A and B. On the other hand, it was 54.2% under schedule C. The overall urinary recovery of CPDD ranged from 25.3 to 35.3% which suggested the retention of CPDD. The urinary excretion of CPDD and the concentration of CPDD in urine showed close relation with a urine volume per hour. The levels of CPDD remained lower in the ascites compared to those of the serum. No significant elevation in the serum creatinine or BUN were recognized in all patients.

Comparison of intermonomer interactions within polymers of chicken gizzard and rabbit skeletal muscle actins

Comparison of interactions between the monomers of chicken gizzard and skeletal muscle actin revealed: 1. A more pronounced increase of the extent of polymerization of chicken gizzard than skeletal muscle actin with temperature, which resulted in higher positive changes of entropy and enthalpy of the former than of the latter species. 2. A difference in spectral changes accompanying polymerization: the changes at 295 nm, attibuted to environmental changes around tryptophan residues, were less pronounced for gizzard than for skeletal actin. 3. A difference in the amount of heavy meromyosin added to gizzard and to skeletal F-actin, with which the degree of flow birefringence of the acto-HMM complex is minimum: this amount was lower in the former than in the latter case. These results indicate quantitative differences between intermonomer interactions involved in polymerization of both actin species and also a possible difference in cooperativity between the monomers within the polymers of gizzard and skeletal actin.