Biochemical maturation of human central nervous system myelin.

—A developmental study of the lipid and protein composition of human CNS myelin was undertaken. The relative concentrations of the major lipid classes, cholesterol, glycolipids and phospholipids exhibited little change except for a modest decrease in the concentration of the phospholipids. In contrast to the total phospholipids, marked variations in the relative concentrations of individual phospholipids were found. Sphingomyelin increased over two-fold, and phosphatidyl choline decreased to almost half its original concentration. While the concentration of total myelin protein remained constant during maturation, variations in the concentrations of individual proteins were observed. Basic protein constituted 8·5 per cent of the total myelin proteins in the newborn brain and increased to about 30 per cent of the protein in the older ages. The concentrations of proteolipid protein and DM-20 seemed to increase with age, while the relative amounts of high molecular weight proteins decreased. The presence of myelin basic protein in newborn human brain was confirmed by electrophoretic studies involving several different polyacrylamide gel systems and by immunodiffusion experiments which showed a reaction of identity between a constituent present in the fraction containing the presumptive myelin basic protein and authentic myelin basic protein isolated from adult human brain.     

CHANGES IN THE PROTEIN COMPOSITION OF MOUSE BRAIN MYELIN DURING DEVELOPMENT

Abstract— Myelin was isolated from the brains of mice at various ages by a procedure involving a final purification on a continuous CsCl gradient. Myelin protein accumulated throughout development, increasing from 0.25 mg of protein/brain at 8 days of postnatal age to 3.5 mg of protein/brain at 300 days, although the rate of accumulation was greatest at about 21 days of age. Quantitative studies of the protein composition of these samples were carried out, utilizing discontinuous polyacrylamide gel electrophoresis in buffers containing sodium lauryl sulphate. Mouse brain myelin, contained (in order of increasing molecular weight) two basic proteins, an uncharacterized doublet, proteolipid protein, and a group of high molecular weight proteins. There were marked changes in the quantitative distribution of these proteins with increasing postnatal age. The basic protein fraction of total myelin protein increased from about 18 per cent at 8 days to 30 per cent at 300 days of age. Proteolipid protein increased even more dramatically, from 7 to 27 per cent in the same time interval. These chemical studies were correlated with ultrastructural investigations, both of the developing myelin sheath in situ and the isolated myelin obtained from mice of various ages. A hypothesis, relating the observed changes in protein composition of myelin during development to its mode of formation, is developed. Another subcellular fraction, separated from myelin, by virtue of its greater density in a CsCl gradient, was also studied. It was a vesicular, membranous fraction present at a level of 0.35 mg of protein/brain at all ages and was related to myelin in terms of protein composition.     

Studies on the Wolfgram High Molecular Weight CNS Myelin Proteins: Relationship to 2'',3''-Cyclic Nucleotide 3''-Phosphodiesterase

Evidence is presented that the major protein components of the high molecular weight CNS myelin proteins designated as the Wolfgram protein doublet (W1 and W2) contain the enzyme 2',3'-cyclic nucleotide 3'-phosphodiesterase (EC 3.1.4.37, CNP). CNP is a basic hydrophobic protein containing about 830 to 840 amino acid residues. When electrophoresed on SDS polyacrylamide gels, CNP appears as a protein doublet, separated by a molecular weight difference of about 2500-3000 in bovine, human, rat, guinea pig, and rabbit. A similar protein doublet has been identified as the Wolfgram proteins W2 and W1 in myelin and in the chloroform-methanol-insoluble pellet obtained from myelin. Moreover, the relative Coomassie blue staining intensity of the CNP2 plus CNP1 protein doublet among the species examined was remarkably similar to that observed for electrophoresed myelin and chloroform-methanol-insoluble pellet derived from myelin. Antisera raised against purified bovine CNP recognized the W1 and W2 proteins isolated from bovine and human brain. The amino acid composition of pure bovine CNP is presented and compared with the compositions of several rat and bovine Wolfgram proteins obtained by other investigators. Our electrophoretic, compositional, and immunological data support the contention that the enzyme CNP is a major component of the Wolfgram protein doublet.     

Bony Fish Myelin: Evidence for Common Major Structural Glycoproteins in Central and Peripheral Myelin of Trout

Peripheral nervous system (PNS) myelin from the rainbow trout (Salmo gairdneri) banded at a density of 0.38 M sucrose. The main myelin proteins consisted of (1) two basic proteins, BPa and BPb (11,500 and 13,000 MW, similar to those of trout central nervous system (CNS) myelin proteins BP1 and BP2), and (2) two glycosylated components, IPb (24,400 MW) and IPc (26,200 MW). IPc comigrated with trout CNS myelin protein IP2 in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, whereas trout CNS myelin protein IP1 had a lower molecular weight (23,000). Following two-dimensional separation, however, both IPb and IPc from PNS showed two components; the more acidic component of IPc comigrated with IP2 from CNS. PNS tissue autolysis led to the formation of IPa (20,000 MW), consisting of two components in isoelectric focusing of which again the more acidic one comigrated with the CNS autolysis product IP0. Limited enzymatic digestion of isolated IP proteins from PNS and CNS led to closely similar degradation patterns, being most pronounced in the case of IP2 and IPc. Immunoblotting revealed that all IP components from trout PNS and CNS myelins reacted with antibodies to trout IP1 (CNS) and bovine P0 protein (PNS) whereas antibodies to rat PLP (CNS) were entirely unreactive. All BP components from trout PNS and CNS myelins bound to antibodies against human myelin basic protein. On the basis of these studies trout PNS and CNS myelins contain at least one common IP glycoprotein, whereas other members of the IP myelin protein family appear closely related. In the CNS myelin of trout the IP components appear to replace PLP.(ABSTRACT TRUNCATED AT 250 WORDS)     

BIOCHEMICAL CHARACTERIZATION OF A MYELIN FRACTION ISOLATED FROM RAT BRAIN AGGREGATING CELL CULTURES

Abstract— Subcellular fractions isolated from rat brain aggregating cell cultures were studied by electron microscopy and showed the presence of typical myelin membranes. The chemical composition of purified culture myelin was similar to the fraction isolated from rat brain in terms of CNP specific activity, protein and lipid composition. The ratio of small to large components of myelin basic protein was comparable in culture and in vivo. These two proteins incorporated radioactive phosphorus. The major myelin glycoprotein was present and during development in culture its apparent molecular weight decreased although it never reached the position observed in myelin isolated from adult rats. In culture, the yield of myelin did not increase substantially between 33 and 50 days and was comparable to that of 15-day-old rat brain. The ratio basic protein to proteolipid protein resembled immature myelin and the cerebroside content was very low. A 'floating fraction' was isolated from the cultures and contained some myelin but mostly single membranes. Although these results indicate that myelin maturation is delayed in vitro this culture system provides substantial amounts of purified myelin to allow a complete biochemical analysis and metabolic studies during development.     

Fish rhabdoviruses: comparative study of protein structure.

Proteins from four fish rhabdoviruses have been studied by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The viruses were: trout viral hemorrhagic septicemia (VHS), infectious hematopoietic necrosis virus (IHN), spring viremia virus of carp (SVC), and the pike fry rhabdovirus (PFR). For the two salmonid viruses (VHS-IHN), gel electrophoresis indicated the proteins, with molecular weights estimated to be 190,000, 80,000, 38,000, 25,000, and 19,000, respectively. The electrophoretic profile of the two other viruses (SVC-PFR) revealed four major proteins with molecular weights of 190,000 80,000 42,000 and 21,000, respectively. In this case a minor component with 50,000 daltons was found. For each virus only one protein was found to be glycosylated, i.e., the one with a molecular weight of 80,000. A major protein (molecular weight between 38,000 and 42,000) was found to be associated with the nucleocapsid. All these results revealed marked similarities in protein structure between the four fish rhabdoviruses and the previously well-characterized members of rhabdovirus group. However, one can distinguish two groups of viruses: the first one is composed of salmonid viruses (VHS and IHN) with a protein structure comparable to that of rabies virus and potato yellow dwarf virus; the second one is composed of carp and pike viruses, having a protein structure very similar to that of vesicular stomatitis virus.     

MEMBRANE PROTEINS OF RAT BRAIN: COMPOSITIONAL CHANGES DURING POSTNATAL DEVELOPMENT

Abstract— Membrane fractions from forebrain of rat were isolated at ages ranging from 5 to 93 days. Among these fractions were total membranes, three fractions isolated by density gradient centrifugation, and three subfractions which consisted of purified myelin and of two supernatant fractions. All membrane fractions showed an increase in protein content during the first postnatal month; however, only the myelin fraction and one of its supernatant fractions showed a prolonged accumulation. Myelin protein increased continually from 0.17 mg/g brain at 15 days to 8.3 mg/g brain at 93 days.
All fractions were analysed for protein composition by sodium dodecyl sulphate polyacrylamide gel electrophoresis. Characteristic changes in protein composition were noted during postnatal development, most of which were pronounced up to the age of 20 days. Among others was a decrease in histones as compared to other proteins, with a concomitant shift in preponderance from the slow- to the fast-migrating histone band. In parallel, other proteins of high molecular weight became more prominent. No myelin could be isolated at 5 and 10 days. The deposition of myelin proteins was parallelled by the appearance of the Wolfgram protein which points to a close correlation of the Wolfgram protein to the process of myelination.     

Cloning, Sequencing, and Phylogenetic Analysis of Complementary DNA of Novel Cytochrome P-450 CYP1A in Japanese Eel (Anguilla japonica)

Complementary DNA of cytochrome P-450 CYP1A, in addition to CYP1A1, has been isolated from Japanese eel (Anguilla japonica) liver treated with 3-methylcholanthrene. The cDNA contained a 5′ untranslated region of 66 bp, an open reading frame of 1554 bp coding for 517 amino acids and a stop codon, and a 3′ untranslated region of 1166 bp. The predicted molecular weight of the Japanese eel CYP1A was approximately 58.5 kDa. The nucleotide sequence exhibited identities with the reported CYP1A1 sequences of 77% for Japanese eel, 75% for rainbow trout, 72% for scup, plaice, and butterfly fish, and 71% for toadfish. The deduced amino acid sequence exhibited identities with the reported CYP1A1 sequences of 78% for Japanese eel, 77% for rainbow trout, 75% for scup, 74% for toadfish, 73% for plaice, and 72% for butterfly fish. The novel eel CYP1A obtained had less similarity to the other teleost CYP1A1 proteins (72%–78%) than that of the eel CYP1A1 (74%–80%). When compared with mammalian CYP proteins, the novel eel CYP1A was more similar to the CYP1A1 proteins (54%–56%) than to the CYP1A2 proteins (50%–53%). The phylogenetic tree of the teleost CYP1A genes constructed using the maximum likelihood method suggested that the novel eel CYP1A is ubiquitous among the Anguilliformes. Received August 25, 2000; accepted November 30, 2000     

ISOLATION AND CHARACTERIZATION OF MYELIN-RELATED MEMBRANES

Abstract— Myelin related membrane fractions from rat brain and spinal cord were isolated from material normally discarded during standard myelin isolation procedures. A fraction which floated on 0.32 M-sucrose (F) and the material released after subjecting the myelin fraction to osmotic shock at two stages in the purification (W₁ and W₂) were characterized. These fractions were subjected to subfractionation on three step discontinuous sucrose gradients. Morphologically, the heavier subfrac-tions of W₁ and W₂ were shown to consist mainly of single membranes and vesicles. Sodium dodecyl sulphate (SDS) polyacrylamide gel electrophoresis showed that, relative to myelin, proteolipid and basic protein were reduced in all subfractions, while the high molecular weight proteins were increased. The specific activity of 2′,3′-cyclic nucleotide 3′-phosphohydrolase (CNP) was up to 2-fold higher than that of myelin in the heavier subfractions of W₁ and W₂. The major myelin-associated glycoprotein was also increased in these subfractions as determined by periodic acid-Schiff staining. Differential centrifugation of the initial tissue homogenate to remove microsomes prior to myelin isolation gave rise to W₁ and W₂ subfractions with a CNP specific activity 3–4 times that of myelin. The high molecular weight proteins and glycoproteins were enriched in these microsome-depleted subfractions, but were qualitatively similar to those of myelin. Some of the membranes in these fractions may be derived from the continuum between the plasma membrane of the oligodendrocyte and compact myelin. Fraction F consisted of small membrane fragments and many vesicles, and was particularly deficient in proteolipid. The specific activity of CNP in fraction F was about the same as myelin, while the major myelin associated glycoprotein could not be detected. Fraction F from normal CNS tissue appears to be similar to the floating fractions previously isolated in larger amounts from pathological brain undergoing edematous demyelination.     

Age- and weight-dependent susceptibility of rainbow trout Oncorhynchus mykiss to isolates of infectious haematopoietic necrosis virus (IHNV) of varying virulence

The virulence of 5 European and 1 North American isolate of infectious haematopoietic necrosis virus (IHNV) was compared by infecting female sibling rainbow trout ('Isle of Man' strain) of different weights and ages (2, 20 and 50 g). The fish were exposed to 10(4) TCID50 IHNV per ml of water by immersion, and the mortality was recorded for 28 d. Two new IHNV isolates from Germany were included in the investigation. One was isolated from European eels kept at 23 degrees C (+/- 2 degrees C) and the other was not detectable by immunofluorescence with commercially available monoclonal antibodies recognising the viral G protein. The results showed that IHNV isolates of high or low virulence persisted in rainbow trout of all ages/weights for 28 d, with the exception of fish over 15 g in the eel IHNV (DF [diagnostic fish] 13/98)-infected groups from which the virus could not be reisolated on Day 28. The smallest fish were most susceptible to an infection with any of the IHNV isolates. The lowest cumulative mortality (18%) was observed in fingerlings infected with the North American isolate HAG (obtained from Hagerman Valley), and the highest mortality (100%) in DF 04/99 infected fish. The DF 04/99 and O-13/95 viruses caused mortality in fish independent of their weight or age. The isolates FR-32/87 and I-4008 were virulent in fish up to a weight of 20 g and caused no mortality in larger fish. In the IHNV HAG- and DF 13/98 (eel)-infected rainbow trout, no signs of disease were observed in fish weighing between 15 and 50 g. An age/weight related susceptibility of rainbow trout was demonstrated under the defined conditions for all IHNV isolates tested.     

Peptidylarginine deiminase gene is differentially expressed in freshwater and brackish water rainbow trout

Peptidylarginine deiminase (PADI)-like cDNA sequence was isolated from rainbow trout (Oncorhynchus mykiss). It consists of a 111-bp 5′-untranslated region, a 731-bp 3′-UTR, and a 2,010-bp open reading frame encoding a protein of 669 amino acids. In the presence of calcium ions, PADI enzymes catalyze the post-translational modification reaction generating citrulline residues. Mammalian PADI enzymes are involved in a number of regulatory processes during cell differentiation and development such as skin keratinization, myelin maturation, and histone deimination. Though five PADI isotypes have been isolated from mammals, in bony fish only one PADI enzyme is present, which contains conserved amino acid residues responsible for catalysis and calcium ion-binding. Sequence identity of piscine PADI protein sequences available at gene databases exceeds 67%. Phylogenetic analyses revealed that not only piscine, but also amphibian and avian PADI-like proteins share most identical amino acid residues with mammalian PADI2. mRNA level of trout PADI-like gene is high in skin, fin, gills, brain, and spleen of rainbow trout. Quantitative Real-Time RT-PCR revealed that PADI gene is differentially expressed in liver, trunk kidney, and spleen of two trout strains, the freshwater-cultured STEELHEAD trout and the brackish water strain BORN.     

ANOMALIES OF MYELIN-ASSOCIATED GLYCOPROTEINS IN''QUAKING MICE

Abstract— Proteins and glycoproteins in a myelin fraction isolated from Quaking mutant mice were separated by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate and stained with Fast Green or with periodic acid-Schiff reagents. Double labelling experiments with [³H]fucose and [¹⁴C]fucose were also used to compare glycoproteins in myelin from the mutant mice with those from control mice. In the myelin fraction from the Quaking mice the basic proteins and proteolipid protein were decreased relative to the high molecular weight proteins. Some glycoproteins which are present in small amounts in myelin from normal mice were increased relative to the major glycoprotein in the myelin fraction of the Quaking mice. Furthermore, the major myelin-associated glycoprotein was shifted toward higher apparent molecular weight in comparison with controls of the same age or even with 9-day-old controls. The abnormal glycoproteins in the mutant myelin fraction could be a factor in the impairment of myelination.     

Biochemical Characterization of Myelin Isolated from the Central Nervous System of Xenopus Tadpoles

Abstract: Myelin isolated from the central nervous system of Xenopus tadpoles was characterized biochemically and compared with Xenopus frog and mammalian myelins. Xenopus tadpole myelin contains the characteristic protein and lipid components of mammalian myelin, although quantitative differences exist. The biochemical composition of Xenopus tadpole myelin suggests that it is an immature form of XePnopus frog myelin. Basic protein and proteolipid protein are prominent components of Xenopus myelin, but isolated tadpole myelin contains a greater proportion of higher molecular weight proteins than Xenopus frog or mature mammalian myelin. The basic protein has a higher apparent molecular weight than mammalian myelin basic protein. The levels of 2',3'-cyclic nucleotide 3'-phosphodiesterase are significantly higher in whole tadpole brain homogenate and purified myelin than in similar mammalian preparations. Tadpole myelin lipids contain a higher proportion of phospholipids and less galactolipid than mammalian myelin. Tadpole myelin galactolipids include a high (16%) percentage of monogalactosyl diglyceride, a component found in only trace quantities (0.9%) in bovine myelin.     

Body composition and organ weights of rainbow trout,Salmo gairdneri*

The objectives of the study were to determine the chemical constituents of rainbow trout (Salmo gairdneri) during the first 14 months of life, to study the effects of starvation on the body composition of trout, and to measure organ weights during the first 14 months of life. Body fat and protein content as a percentage of body weight increased with age, the water content declined, and the ash content remained constant during the first 14 months of life. There were no significant differences in body composition between immature male and female rainbow trout. Total body ions as a proportion of ash content decreased until the 10th month and then began to increase. During starvation, fat was used as the primary source of energy while water and ash content increased and protein content remained constant as a percentage of body weight. The gall bladder, liver, and air bladder remained relatively constant as a percentage of body weight during the first year of life. As body weight increased, the relative weight of brain, heart, and digestive system decreased while the gonad and spleen weight increased.     

Major Central Nervous System Myelin Glycoprotein of the African Lungfish (Protopterus dolloi) Cross-Reacts with Myelin Proteolipid Protein Antibodies, Indicating a Close Phylogenetic Relationship with Amphibians

CNS myelin was isolated from the spinal cord of the African lungfish Protopterus dolloi. Its proteins consisted of (1) two basic proteins (16,000 and 18,500 apparent Mr) that reacted with anti-human CNS myelin basic protein antibodies and (2) a major protein (29,000 apparent Mr) that stained with concanavalin A-horseradish peroxidase and bound to anti-rat CNS myelin proteolipid protein (PLP) antibodies. This dominant 29,000 Mr protein showed no reaction with antibodies against the major bovine PNS myelin glycoprotein P0. Following treatment with endoglycosidase F the 29,000 Mr protein was reduced in size to a 26,000 apparent Mr component that no longer bound concanavalin A but retained the anti-PLP reactivity. These results agree with a concanavalin A-binding oligosaccharide linked through asparagine to a protein backbone of PLP homology. The major 29,000 Mr lungfish CNS myelin protein was therefore termed g-PLP (glycosylated proteolipid protein). This is the first report demonstrating the occurrence of a PLP-cross-reactive protein in CNS myelin of a fish. It attests to the close phylogenetic relationship of lungfishes to amphibians. Amphibians were previously recognized as the oldest class bearing PLP in its CNS myelin.     

Identification and Topography of Substrates for Protein Carboxyl Methyltransferase in Synaptic Membrane and Myelin-Enriched Fractions of Bovine and Rat Brain

The major components of crude brain synaptosomes (synaptic membranes, mitochondria, and myelin) have been separated and analyzed by polyacrylamide gel electrophoresis for the presence of proteins that serve as substrates for protein carboxyl methyltransferase. Of the three fractions, synaptic membranes contain the largest number of individual methyl acceptors (at least seven), while mitochondria contain no well-defined methyl acceptors. Undisrupted myelin contains a single major methyl acceptor with a very low apparent molecular weight. The patterns of protein methylation in synaptic membranes prepared from cerebral cortex, hippocampus, striatum, thalamus, and tectum showed marked differences; however, these differences could largely be explained by differential degrees of myelin contamination in synaptic membranes from the different regions. The effect of trypsin pretreatment on the carboxyl methylation of intact and lysed synaptosomes was studied to estimate the sidedness of the major methylation sites on synaptic membranes. One of the methyl acceptors (Mr 48K) appears to be facing the intracellular surface of the synaptosome, but most sites appear to be outward facing.     

Synthesis of the myelin proteolipid protein in the developing mouse brain

Mice ranging in age from 16 to 44 days were injected intracerebrally with ³H-leucine, and incorporation into total brain proteolipids and the myelin proteolipid protein was measured. All proteolipids were isolated from whole brain by ether precipitation and separated into their individual components by SDS polyacrylamide gel electrophoresis. Two major proteolipids with apparent molecular weights of 20,700 and 25,400 were observed in these preparations, and their proportion increased over the developmental period examined. A Ferguson plot analysis comparing these proteins with those of isolated myelin showed that the 25,400-dalton proteolipid component from whole brain was the myelin proteolipid protein. Rates of incorporation of ³H-leucine into total brain proteolipids peaked at 22 days of age. Synthesis of the myelin proteolipid protein increased rapidly to a maximum value at 22 days and decreased rather slowly until at 44 days it was about 83% of its maximum rate of synthesis. The data indicate that the developmental pattern of synthesis of the myelin proteolipid protein is unlike that of the myelin basic proteins. Synthesis of the major myelin proteins is developmentally asynchronous in that peak synthesis of the myelin proteolipid appears to occur several days later than the basic proteins. In addition, it maintains its maximum rate of synthesis over a longer period of time than do the basic proteins.     

Advancing provision of multi-species fish passage: Behaviour of adult European eel (Anguilla anguilla) and brown trout (Salmo trutta) in response to accelerating flow

The behavioural response of downstream moving European eel (Anguilla anguilla) and brown trout (Salmo trutta) to velocity gradients created by orifice weirs placed in a flume were studied. The aim was to identify interspecific variation and test two hypotheses: (1) a more abrupt velocity gradient would induce a higher avoidance response and over a greater distance, and (2) fish would acclimate to the velocity gradient by incrementally sampling conditions closer to the orifice until successful passage occurred. European eel moved along the channel floor and walls, responding after physical contact with the weir. Brown trout moved downstream head first and switched orientation at the velocity gradient without contacting the structure. Brown trout spent longer than European eel immediately upstream of a channel floor orifice, although time to pass was similar; but less time at a mid-column orifice despite taking longer to pass. The mid-column orifice delayed both species longer. European eel passed orifices head first on initial encounter with no rejections, whereas brown trout passed tail first. Positions where brown trout first switched orientation and closest point to the orifice reached during initial approach occurred closer to a channel floor than mid-column treatment. Brown trout did not appear to acclimate to the velocity gradient by sampling successively closer to the orifice. Interspecific variation in behaviour remains a key challenge in developing multi-species fish passes.     

Cloning and Sequencing of Cytochrome P450 1A Complementary DNA in Eel (Anguilla japonica)

Cytochrome P450 1A (CYP1A) complementary DNA was isolated from eel (Anguilla japonica) liver treated with 3-methylcholanthrene. The cDNA contained a 5′ untranslated region of 163 bp, an open reading flame of 1560 bp coding for 519 amino acids and a stop codon, and a 3′ untranslated region of 1730 bp. The predicted molecular weight was approximately 58.4 kDa. The deduced amino acid sequence exhibited identities with reported CYP1A sequences of 80% for rainbow trout, 79% for scup, 76% for plaice and butterfly fish, and 74% for toadfish. When compared with mammalian CYP proteins, the eel CYP1A was more similar to CYP1A1 (54%–56%) than to CYP1A2 (49%–52%). Northern and Southern blot analyses showed two distinct bands, suggesting the existence of another 3-methylcholanthrene-inducible CYP1A gene in eel. Received December 19, 1998; accepted February 18, 1999     

Mouse brain protein composition during postnatal development: an electrophoretic analysis

Changes in the concentrations of mouse brain proteins during postnatal maturation were characterized by a combination of subcellular fractionation and electrophoresis. Sodium dodecyl sulfate gel electrophoresis revealed changing protein concentrations in fractions enriched in nuclei, mitochondria plus synaptic endings, microsomes and cytosol. Postnatal maturational changes in protein concentrations were most pronounced in fractions of purified myelin membranes. The use of exponential gradient gels resulted in increased resolution of low molecular weight myelin proteins. Nuclei treated with Triton X-100 exhibited no change in relative histone concentrations during brain maturation. Nonnuclear contamination of untreated nuclear fractions was shown to be a potential source of erroneous interpretations. These findings are discussed in terms of genetic products and sodium dodecyl sulfate polyacrylamide gel electrophoresis resolution.