Hemoglobin synthesis during amphibian metamorphosis. I. Chemical studies on the hemoglobins from the larval and adult stages of Rana catesbeiana

Investigating Viruses during the Transformation of Molecular Biology

This Reflections article describes my early work on viral enzymes and the discovery of mRNA capping, how my training in medicine and biochemistry merged as I evolved into a virologist, the development of viruses as vaccine vectors, and how scientific and technological developments during the 1970s and beyond set the stage for the interrogation of nearly every step in the reproductive cycle of vaccinia virus (VACV), a large DNA virus with about 200 genes. The reader may view this article as a work in progress, because I remain actively engaged in research at the National Institutes of Health (NIH) notwithstanding 50 memorable years there.

ERYTHROPOIESIS DURING AMPHIBIAN METAMORPHOSIS : II. Immunochemical Study of Larval and Adult Hemoglobins of Rana catesbeiana

Rabbit antibodies were prepared against the major hemoglobin components of the larval and adult stages of R. catesbeiana. The properties of the antisera were studied by double immunodiffusion, precipitation, and complement fixation. The antisera to tadpole and frog hemoglobins did not cross-react with either hemoglobin or apohemoglobin. The anti-serum against frog hemoglobin was used for the detection of frog hemoglobin in tadpoles undergoing either natural or thyroxine-induced metamorphosis. It was shown that frog hemoglobin is detectable first in the liver, indicating that the liver is the site of erythrocyte maturation during metamorphosis.

ERYTHROPOIESIS DURING AMPHIBIAN METAMORPHOSIS : III. Immunochemical Detection of Tadpole and Frog Hemoglobins (Rana catesbeiana) in Single Erythrocytes

Rabbit antibodies specific for the major tadpole and frog hemoglobin components of R. catesbeiana were used for the detection of the two hemoglobins inside single cells. The antisera, after fractionation by ammonium sulfate precipitation and diethylaminoethyl (DEAE)-cellulose chromatography, were conjugated with fluorescein isothiocyanate for the antifrog hemoglobin serum and tetramethylrhodamine isothiocyanate for the antitadpole hemoglobin serum. The conjugated fractions, refractionated by stepwise elution from a DEAE-cellulose column, were used for the fluorescent staining of blood smears, liver tissue imprints, and smears of liver cell suspensions. Both simultaneous and sequential staining with the two fluorescent preparations indicated that larval and adult hemoglobins were not present within the same erythrocyte during metamorphosis. In other experiments, erythroid cells from animals in metamorphosis were spread on agar containing specific antiserum. Precipitates were formed around the cells which contain the particular hemoglobin. The percentages of cells containing either tadpole or frog hemoglobin were estimated within the experimental error of the method. The data showed that the two hemoglobins are in different cells. It is concluded that the hemoglobin change observed during the metamorphosis of R. catesbeiana is due to the appearance of a new population of erythroid cells containing exclusively frog hemoglobin.

A new hemoglobin beta chain variant in sheep

A new hemoglobin beta chain variant referred to as Hb-E is observed in four Indian sheep breeds. Hemoglobin E is electrophoretically and chromatographically undistinguishable from hemoglobin B. However, polypeptide chain separation by urea-polyacrylamide disc gel electrophoresis and urea CM-cellulose chromatography reveals the new beta chain. The betaE chain has a slower anodic mobility than betaB chain at alkaline pH. The adult sheep hemoglobins could be classified into eight categories, namely AA, AB, AE, AC, ABC, BB, BE and EE, according to their sub-unit composition. The occurrence of betaE chain in homozygous condition or in combination with either one of the two allelic beta chains betaA or betaB, indicate that it may be an allelic variant. Gene frequencies of three hemoglobin beta chain alleles observed during the study are as follows: A = 0.192; B= 0.682; E = 0.126.

Axolotl hemoglobin: cDNA-derived amino acid sequences of two α globins and a β globin from an adult Ambystoma mexicanum

Erythrocytes of the adult axolotl, Ambystoma mexicanum, have multiple hemoglobins. We separated and purified two kinds of hemoglobin, termed major hemoglobin (Hb M) and minor hemoglobin (Hb m), from a five-year-old male by hydrophobic interaction column chromatography on Alkyl Superose. The hemoglobins have two distinct alpha type globin polypeptides (alphaM and alpham) and a common beta globin polypeptide, all of which were purified in FPLC on a reversed-phase column after S-pyridylethylation. The complete amino acid sequences of the three globin chains were determined separately using nucleotide sequencing with the assistance of protein sequencing. The mature globin molecules were composed of 141 amino acid residues for alphaM globin, 143 for alpham globin and 146 for beta globin. Comparing primary structures of the five kinds of axolotl globins, including two previously established alpha type globins from the same species, with other known globins of amphibians and representatives of other vertebrates, we constructed phylogenetic trees for amphibian hemoglobins and tetrapod hemoglobins. The molecular trees indicated that alphaM, alpham, beta and the previously known alpha major globin were adult types of globins and the other known alpha globin was a larval type. The existence of two to four more globins in the axolotl erythrocyte is predicted.

A comparative study on earthworm hemoglobins: an amino acid sequence comparison of monomer globin chains of two species, Pontodrilus matsushimensis and Pheretima communissima that belong to the family Megascolecidae

The monomer subunits of giant extracellular hemoglobins from earthworms Pontodrilus matsushimensis and Pheretima communissima that belong to the family Megascolecidae, Oligochaeta, were purified by a reversed-phase column, Resource RPC, and sequenced. The complete amino acid sequences of the two monomeric globin chains were determined: 141 amino acid residues with a molecular weight of 16,366 Da for Pontodrilus matsushimensis and 140 amino acid residues with a molecular weight of 16,000 for Pheretima communissima, respectively. The Pontodrilus matsushimensis monomer globin has three cysteine residues, and the two located at positions 2 and 131 are conserved as those observed in all annelids and contribute to form a disulfide-bonded interchain. The third cysteine residue at position 73 is the first evidence for the annelid monomer globin subunits. The physiological functions of the third cysteine residue, however, are still unknown. The monomer sequences of the two species were aligned with those of five known sequences from annelids, including a polychaete, Tylorrhynchus heterochaetus, and four oligochaetes, Pheretima hilgendorfi, Pheretima sieboldi, Lumbricus terrestris and Tubifex tubifex. Using computer analysis, a 87.9% identity of the amino acid sequences between two monomeric subunits of Pheretima communissima and Pheretima hilgendorfi hemoglobins showed the highest degree of sequence similarity. A molecular phylogenetic tree of seven species of annelids has constructed, suggesting that the divergence times among the three species of Pheretima and between Pheretima and Pontodrilus were 50 to 100 and about 209 million years ago, respectively.

Amino acid sequence of the monomer subunit of the extracellular hemoglobin of the earthworm, Pheretima hilgendorfi

The complete amino acid sequence of the monomer subunit of Pheretima hilgendorfi hemoglobin was determined: It consists of 140 amino acid residues, including a disulfide bond but no methionine, and has a molecular weight of 16,107 Da. Using computed analyses (amino acid maximum homology) with known sequences of monomer subunits of earthworm's hemoglobins, 115 (82%) were found to be identical with those in the corresponding positions of chain I (monomer subunit) of Pheretima sieboldi hemoglobin; 81 residues (55%), 71 residues (47%), and 66 residues (43%) were found to be in identical positions of the sequences of chain I of Lumbricus terrestris hemoglobin, chain I of Tubifex tubifex hemoglobin and chain I of Tylorrhynchus heterochaetus hemoglobin. Orthologous sequence data of monomer globins that belong to the strain A of annelid hemoglobins are discussed as useful clues for investigation of the divergence pattern of Pheretima species.

Haemoglobin I: a new beta-globin chain variant found in sheep of Italian breeds

A rather common haemoglobin variant was detected in the Sardinian and Altamurana sheep breeds. The mutated globin chain appears to be produced under the control of an allele at the HBB locus and due to a neutral amino acid substitution. The variant will be provisionally referred to as the Hb I.

Coexpression of embryonic, fetal, and adult globins in erythroid cells of human embryos: relevance to the cell-lineage models of globin switching

The cellular control of the switch from embryonic to fetal globin formation in man was investigated with studies of globin expression in erythroid cells of 35- to 56-day-old embryos. Analyses of globins synthesized in vivo and in cultures of erythroid progenitors (burst-forming units, BFUe) showed that cells of the yolk sac (primitive) erythropoiesis, in addition to embryonic chains, produced fetal and adult globins and that cells of the definitive (liver) erythropoiesis, in addition to fetal and adult globins, produce embryonic globins. That embryonic, fetal, and adult globins were coexpressed by cells of the same lineage was documented by analysis of globin chains in single BFUe colonies: all 67 yolk sac-origin BFUe colonies and 42 of 43 liver-origin BFUe colonies synthesized epsilon-, gamma-, and beta-chains. These data showed that during the switch from embryonic to adult globin formation, embryonic and definitive globin chains are coexpressed in the primitive, as well as in the definitive, erythroid cells. Such results are compatible with the postulate that the switch from embryonic to fetal globin synthesis represents a time-dependent change in programs of progenitor cells rather than a change in hemopoietic cell lineages.

Hemoglobin transition in the anuran Pelodytes punctatus

In the anuran Pelodytes punctatus the larval hemoglobin produces five electrophoretic bands. In the premetamorphic period, two other bands appear which are typical of the adult. They gradually substitute the larval bands completely as in other anurans. The pathway of the hemoglobin shift indicates that the synthesis of the adult fractions does not depend on the thyroid hormones for its activation, whereas the disappearance of the larval fractions depends on the destruction of the "larval" red blood cell line. As in other species of anurans which are considered primitive, the hemoglobinic change of Pelodytes punctatus starts earlier and develops slower than in other more evolved anurans. In normal developing specimens of Pelodytes punctatus the change in the hemoglobin fractions occurs very slowly and finishes 4 weeks after the metamorphosis, whereas in "hibernated" specimens the rate of change is higher and it ends earlier, probably as a consequence in the retarded larvae, of the increased sensibility of the hemopoietic tissues to the metamorphosis factors.

Hemoglobin heterogeneity in the pike, Esox lucius, and isolation of a homogeneous component

The hemolysate of the pike, Esox lucius, contains multiple molecular forms of hemoglobin molecules. Analytical gel electrofocusing, disc-gel electrophoresis and gel filtration chromatography indicate that the hemoglobin heterogeneity may be due to different charge isomers with mol. wt values similar to that of the human. Ion exchange chromatography on CM-cellulose permits to separate a hemoglobin component in a pure form. Globin chains analysis of this fraction and their isolation was also reported.

Multiple hemoglobins in Triturus cristatus: their study by analytical isoelectrofocusing

Electrophoretic separation of the hemoglobin of healthy adult Triturus cristatus reveals four components. Isoelectric focusing of the same hemolysates in various commercial ampholytes of different chemical composition and pH range results in the separation of eight individual hemoglobin bands. The bands obtained by electrophoresis are not homogeneous as revealed by individual gel electrofocusing. They finally separate into the same eight components, as in the whole hemolysate. From the above findings it is concluded that this species has not four but eight individual hemoglobin molecular forms. Our results demonstrate lack of hemoglobin polymorphism in this species.

Tadpole erythrocytes: luminescent properties with dark field microscopy

To characterize and classify erythrocytes of ranid tadpoles, alcohol-fixed blood smears were studied with dark field illumination. All preclimax stage (limb bud-foot) Rana castesbeiana tadpoles from ponds in Massachusetts had red blood cells that were polymorphic. The majority of cells (88%) showed a bright, granular luminescence varying from white to blue-grey, whereas, cytoplasm of the other cells was smooth, black, and nonluminescent. On the other hand, tadpoles in similar stages from other species (Rana clamitans, Mass. and Rana pipens, Vermont) and from R. catesbeiana tadpoles from other locations (Wisconsin and North Carolina) had no observable cytoplasmic luminescence in any of their red blood cells. Moreover, as Mass. R. catesbeiana underwent metamorphic climax their luminescent cells disappeared and were replaced by small, round, dark, nonluminescent cells, precursors of the oval, nonluminescent erythrocytes characteristic of adult frogs. Cells with black nonluminescent cytoplasm generally contained nuclei which were luminescent. In conclusion, two main types of red blood cells-those with and those without cytoplasmic luminescence-are distinguishable with dark field microscopy. Luminescence of the cells varies with species, geographic location, and developmental stage of the tadpoles.

Globin gene expression in erythroid cell lines during larval development of Pleurodeles waltlii

We have attempted to determine whether in Pleurodeles ontogenesis there exists a close relationship between the two following characteristics: change from primitive to definitive erythroid cell populations, which parallels the change of major erythropoietic site; change in the type of synthesized hemoglobin, larval or adult. The origin of red blood cells was investigated by embryonic grafts of hemopoietic anlage from 2n to 4n embryos. The larval or adult hemoglobin type was characterized by immunofluorescence by using specific antibodies. Our results show that in Pleurodeles, blood island-originating red blood cells and spleen-originating red blood cells are both able to synthesize either Hb L or Hb A at a given time, but in separate cells.

Cell surface proteins of Drosophila. II. A comparison of embryonic and ecdysone-induced proteins

The cell-surface proteins of Drosophila embryos at gastrula and myoblast fusion stages were characterized by radioiodination and two-dimensional gel electrophoresis. Over 13% of the cell surface proteins detected in gastrula embryos were not found in myoblast fusion stage embryos or in Drosophila embryonic cell line EH34A3 cells. Nearly 18% of the cell-surface proteins detected in myoblast fusion stage embryos were evident only at that stage. Embryonic cell-surface proteins were compared with cell-surface proteins from untreated EH34A3 cells and EH34A3 cells treated with 20-hydroxyecdysone, which induces cell aggregation and the expression of "new" proteins at the cell surface (D. F. Woods and C. A. Poodry, 1983, Dev. Biol. 96, 23-31). Only one of the proteins induced by ecdysone in EH34A3 cells was detected in the NP-40 soluble fraction of radioiodinated cell lysates, even after fractionation by lectin affinity chromatography and immunoprecipitation to enrich for putative ecdysone induced proteins. However, extraction of the NP-40 insoluble pellet of embryo cells revealed one additional protein that was present both in myoblast fusion stage embryos and hormone-treated culture cells. It was concluded that except for these two proteins, the cell-surface proteins induced in cultured cell lines by treatment with 20-hydroxyecdysone are not present in significant amounts in gastrula or myoblast fusion stage embryos.

Embryonic and larval hemoglobins during the early development of the bullfrog, Rana catesbeiana

The main hemoglobin (Hb) found in Shumway (embryonic) stage 25 bullfrogs is that which we have designated Td-4. The other major tadpole Hbs (Td-1, 2, and 3) predominate during Taylor and Kollros (larval) stages I-XVIII. We propose that Td-4 is an embryonic Hb, whereas Td-1, 2, and 3 are larval (fetal-like) Hbs. Embryonic Hb Td-4 continues to be synthesized during the larval stages. During the larval period, the average peripheral blood Hb profile changes very little with morphological stage or general growth. However, there is great heterogeneity in the embryonic:larval Hb ratio among individual tadpoles of a given stage or weight, apparently due to differential Hb and red cell production by the two active erythropoietic sites, mesonephric kidneys (Td-4), and liver (Td-1, 2, 3).

Developmental time of the hemoglobin transition in the anuran Bombina orientalis

The electrophoretic pattern of the larval hemoglobin of the anuran Bombina orientalis presents two bands. In premetamorphic period gradually appear four other bands, corresponding to those of the adult hemoglobin; they substitute, within 15-16 days after metamorphosis, the larval pattern. The percentage of larval and adult fractions during the development is shown. In Bombina orientalis the change of the hemoglobinic fractions from larval to adult type is total as in most anurans. But it starts earlier than in other species studied and develops slower than most of the other species.

Bile pigments and bilirubin UDP-glucuronyl transferase during the metamorphosis of Rana catesbeiana tadpoles

The major bile pigments in Rana catesbeiana tadpoles was bilirubin IX alpha. The concentration of bilirubin IX alpha increases in bile and plasma during metamorphosis. Bilirubin IX alpha and biliverdin IX alpha were also present in the bile of tadpoles. Bilirubin UDP-glucuronyl transferase activity was present in the livers of all tadpoles examined. Bilirubin UDP-glucuronyl transferase activity increases slightly during spontaneous metamorphosis and increases approximately 2-fold during T3-induced metamorphosis.

Urea polyacrylamide gel electrophoresis of PCMB precipitate as a sensitive test for the detection of the unstable hemoglobin subunit

A sensitive test for the detection of the abnormal subunit of unstable hemoglobins is described. The unstable hemoglobin subunit was selectively precipitated by paramercuribenzoic acid treatment of the carboxy hemolysate, and the precipitate was studied by urea polyacrylamide gel electrophoresis for globin composition. The test enabled the detection of an electrophoretically silent, alpha-chain unstable variant comprising only 1% of total hemoglobin. Other applications included the demonstration of Hb Köln in a cord blood, and the detection and purification of very slightly unstable hemoglobin variants.

Changes in the density of circulating erythrocytes of the bullfrog tadpole, Rana catesbeiana, in relation to the transition of hemoglobin during metamorphosis

1. The circulating erythrocytes of Rana catesbeiana tadpoles at varying metamorphic stages were studied by the phthalate-microcapillary technique of Danon & Marikovsky (1964 J. Lab. Clin. Med. 64, 668-674) and by the method of Murphy (1973 J. Lab. Clin. Med. 82, 334-341), which are used for the separation of erythrocytes according to the density. 2. The erythrocytes of tadpoles were different in the mean density from those of adult frogs. 3. At the metamorphic climax, aged erythrocytes having the tadpole hemoglobin coexisted with young erythrocytes having the frog hemoglobin in the circulating blood. 4. The frog hemoglobin was preferentially synthesized during metamorphosis. 5. All the results suggested that the transition of hemoglobin during metamorphosis was associated with the replacement of erythrocyte population.

Comparison of hemoglobins from the genus Xenopus (Amphibia salienta)

A survey of the electrophoretic mobility of hemoglobins of 14 species and subspecies of the genus Xenopus was undertaken. It was found that the variation between the different taxons considered was quite important in the number and the degree of the hemoglobin bands on polyacrylamide slab gels. The profiles of the hemoglobin electrophoretic pattern in tube gels gave corresponding results, confirming the variation between the different Xenopus species. The degree of resolution obtained on polyacrylamide slab gels was insufficient to allow one to distinguish between the subspecies of Xenopus laevis. Specific staining of the gels showed that the protein bands were in fact hemoglobins. From the results obtained it appears that there are relative differences in the number and mobility of hemoglobin bands between the different species, these differences being important enough to stimulate interest in further investigation in this field, but, as far as this preliminary investigation indicates, not clear-cut enough to use as a method of identification for isolated unknown species.

Differential effects of pH and inositol hexaphosphate on the spectroscopic properties of the alpha and beta subunits in methemoglobins M Milwaukee and A

The effect of pH and inositol hexaphosphate on the electron spin resonance spectra of the alpha-hemes (g = 6.0) and the beta-hemes (g = 6.7) has been measured in methemoglobin M Milwaukee and compared with that of methemoglobin A (g = 6.0). The beta-hemes are found to be comparatively insensitive to both effectors while the alpha-hemes behave in a manner similar to the heme groups of methemoglobin A. Binding of inositol hexaphosphate enhances the high spin ESR signal of the alpha-hemes in both methemoglobins. Comparison of the optical properties of methemoglobins A and M Milwaukee over the pH range from 5.0 to 8.1 shows that inositol hexaphosphate has a differential effect on the subunit types in these two methemoglobins. At low pH the spectral changes observed upon inositol hexaphosphate binding arise primarily from the beta-hemes, while at neutral and alkaline pH these changes arise from both subunit types. The beta-heme spectral changes appear to be pH independent while those arising from the alpha-hemes are strongly pH dependent. It is concluded that it is the hydroxymet form of the alpha-hemes which undergoes spectral change upon inositol hexaphosphate binding to the beta-subunits. In methemoglobin A the spin state and paramagnetic susceptibility increase only in the neutral and alkaline pH ranges upon inositol hexaphosphate binding (Gupta, R.K. and Mildvan, R.S. (1975) J. Biol. Chem. 250, 246; Perutz, M.F., Sanders, J.K.M., Chenery, D.H., Noble, R.W., Penelly, R.R., Fung, L.W.-M., Ho, C., Giannini, I., Porschke, D. and Winkler, H. (1978) Biochemistry 17, 3640). Therefore the hydroxymet form of the alpha-hemes which is responsible for the observed spectral changes must also be responsible for these increases in the magnetic properties of methemoglobin A. Inositol hexaphosphate can bind to methemoglobin at alkaline pH if the beta-hemes are in the high spin form.

Identification and characterization of a 19-S complex containing a 27 000-Mr protein in Artemia salina

The cytoplasm of the cryptobiotic Artemia salina gastrula contains a large quantity of a unique 19-S complex. This particle is a specific aggregated form of a 27 000-Mr protein, having a molecular weight of 10(6) and an apparent buoyant density of 1.25 -- 1.26 g/cm3 in sucrose and 1.31 g/cm3 in CsCl. The relative quantity of this 19-S complex decreases significantly with respect to 80-S monoribosomes during the postgastrula development. Biochemical and immunological studies indicate that the 27 000-Mr protein is one of the RNA-binding proteins [Ovchinnikov et al., FEBS Lett. 88, 21 -- 26 (1978)] but is absent in the protein components associated with poly(A)-containing ribonucleoprotein complexes. The possibility is also suggested that the 27 000-Mr protein and Artemia elongation factor eEF-Ts might be related to each other on the basis of amino acid composition and immunological cross-reactivity.

Metamorphic variations in the hemoglobins of Bombina variegata (L.)

1. The electrophoretic pattern of the anuran Bombina variegata presents one band only until the end of premetamorphosis. Afterwards three new bands appear which substitute the larval band within 15-18 days of metamorphosis. 2. The gradual change of larval and adult hemoglobin, correlated with the development, has been reported in a diagram. 3. In Bombina variegata the hemoglobin change is total, like in most amphibians, but starts earlier and develops slower than in most other species.

Gene diversity of bovid hemoglobins

Forty-five adult hemoglobin molecular forms which include 22 electrophoretically silent forms were structurally characterized from animal species of nine genera belonging to family Bovidae. Of the 12 different bovid species studied, 11 showed either alpha or beta chain heterogeneity in their hemoglobins while eight species showed heterogeneity for both polypeptide chains. Wherever possible, the genetic basis for hemoglobin phenotypes has been suggested. By construction of a phylogenetic tree for 14 ungulate alpha-globin sequences, the evolutionary origins of duplicated alpha chain genes present in some ungulate species have been located, and the phyletic relationship of Bovids based on the alpha globin data is discussed.

Globin chain separation by SDS polyacrylamide gel electrophoresis. Simple screening method for elongated hemoglobin chains

A simple method for the separation of hemoglobin chains from hemolysate or globin, by sodium dodecyl sulfate polyacrylamide gel electrophoresis, is described. The alpha, beta, and gamma chains can be clearly separated from each other. The alpha chain has the highest mobility, the beta chain has a slower mobility than the gamma chain, while the delta chain has about the same mobility as the beta chain. Hemoglobins with elongated chains can easily be detected by this method. Tak-beta, elongated by 11 residues, moves much more slowly than betaA but is much faster than alpha Constant Spring which is elongated by 31 residues. Screening of several individuals with slow-moving hemoglobins using this method led to the finding of a case with Hb Tak-beta thalassemia and other carriers of Hb Tak.

Evidence for a dimeric form of Artemia salina extracellular hemoglobins with high-molecular-weight subunits

The brine shrimp, Artemia salina, produces at least three chemically and ontogenetically distinct extracellular hemoglobins (Hb-I, Hb-II and Hb-III). The estimated molecular weights of these hemoglobins are 240000-260000, containing 14 heme groups based on the iron and heme contents of a molar species. Hb-II, which corresponds to a minimal molecular weight of about 18 000 per heme [Moens, L. and Kondo, M. (1977) Biochem. J. 165, 111-119]. Denaturation of the reduced and alkylated hemoglobins with 8 M guanidine hydrochloride revealed apparently one polypeptide chain having a molecular weight of 126 000. Thus a single native hemoglobin molecule should be composed of two of these high-molecular-weight subunits each of which is bound with seven hemes. Upon sodium dodecyl sulfate/polyacryamide gel electrophoresis of either native hemoglobins or isolated subunits it was found that the 126 000-Mr polypeptide was cleaved specifically into two unequally-sized fragments of Mr 50 000 and 80 000. Further denaturation of native hemoglobins with urea at pH 2.5 or 11 followed by sodium dodecyl sulfate gel electrophoresis confirmed these results. The amino acid compositions determined for native Hb-II and its subunit and fragments are found to be very similar, implying that no specifically localized amino acid sequences are present and that the subunit globin chain could be composed of seven similar repeat units (Mr approximately 18 000) being linked covalently to one another. The amino acid compositions of Hb-I and Hb-III showed only minor differences to that of Hb-II.