Old and new genetics help ordering loci at the telomere of the human X-chromosome long arm

Summary A Sardinian pedigree described in 1964 for having been found to segregate at the X-linked loci for the Xg^a antigen, G6PD deficiency, Protan and Deutan color blindness, with an instance of recombination between the last two loci, was re-examined with respect to four common X-linked DNA polymorphisms detected by molecular probes homologous to critical subregions of the human X chromosome. Two branches of this pedigree-including the one with the Protan-Deutan recombinant-were found to segregate also for the common BamHI polymorphism identified with the cDNA probe pHPT-2 of the HPRT gene (Xq26). The analysis of the chromosome haplotypes in the male offspring of the phase known penta-heterozygous mother suggests that the probable order of the relevant loci is HPRT, Deutan, G6PD, Protan, Xq telomere. Though we are fully aware of the risks of generalizing the significance of observations made on a single exceptional pedigree, we believe that this report outlines the potential of families of the type described as reasearch tools to resolve the linear order of tightly X-linked loci and to investigate the biology of genetic recombination in humans.     

Metabolism of prostacyclin. Oxidation by rhesus monkey lung 15-hydroxyl prostaglandin dehydrogenase.

Partially purified rhesus monkey lung 15-hydroxyl prostaglandin dehydrogenase (15-OH PGDH) catalyzed the NAD-dependent oxidation of prostacyclin (PGI₂) and 6-keto-PGF_1a to 6,15-diketo-PGF_1a. The product was identified by gas chromatography-mass spectroscopy. Prostacyclin was oxidized four to six times faster than 6-keto-PGF_1a under identical reaction conditions, suggesting that the metabolism of prostacyclin probably proceeds through a bicyclic 15-keto intermediate before chemically decomposing to the final stable product, 6,15-diketo-PGF_1a. Prostacyclin has a good affinity for the 15-OH PGDH enzyme. A Lineweaver-Burke plot gave an apparent K_m value of 7.4 μm, which compares very favorably with the K_m values for PGE₁ and PGE₂.     

Distribution of hereditary blood groups among Indians in South America. I. In Ecuador

Blood specimens were procured from 658 Quechua, 36 Colorado, 233 Jivaro, 244 Cayapa, and 48 Secoya Indians of Ecuador. These were examined for antigens in the A-B-O, M-N-S-s, P, Rh-Hr, Lutheran, K-k, Lewis, Duffy and Kidd systems and for Diego (Di^a), Wright (Wr^a), and Berrens (Be^a) agglutinogens as well. Hemolystes were prepared and studied for hemoglobin types and the serum samples were tested for haptoglobins and transfserrins. Gene frequencies are high for O, M, s, R¹, (CDe), R² (cDE), Lu^b, k, Kp^b, Le^b and Fy^a; and low or absent for A, B, N, S, Mi^a, Vw, Mt^a, R⁰ (cDe), V (ce^s), Lu^a, K, Kp^a, Le^a, Fy^b, Js^a, Wr^a and Be^a. The Diego (Di^a) gene is present but its frequency varies greatly from tribe to tribe. Gene frequency Hp¹ is well within the range previously reported for Indians in Middle America excepting the Colorado in which population the frequency of 0.889 is unusually high. All 723 serum specimens tested for transferrins were C or CD. No D or BC types were found. All Ecuadorian Indian bloods tested electrophoretically contained only hemoglobin (A) as a major component.     

Spontaneous and induced reversion rates in a double heterozygous mutant of Nicotiana tabacum var. xanthi n.c. —Dose-response relationship

The ability of the double heterozygous mutant a₁⁺/a₁a₂⁺/a₂(greenish-yellow) to revert phenotypically towards green was used to quantify the dose-response relationship at 0, 8, 16, 32, 64, 128 and 256 R of gamma-rays from a ⁶⁰Co-source. All the studies undertaken up to now on the genetic nature of the events responsible for the phenotypic reversion suggest that the changes take place mostly at locus a₁⁺−a₁.The present experiment was split into two parts: a low-dose (0–64 R) and a higher-dose (64–256 R) analysis, in which the responses were quadratic and linear, respectively.The spontaneous reversion rate was estimated to be 10⁻⁵.     

Oxidative metabolism of hydralazine. Evidence for nitrogen centered radicals formation

The oxidative metabolism of hydralazine, a hydrazine-containing hypotensive drug, has been studied using a spin-trapping technique. In the presence of Cu²⁺, Fe²⁺ and Fe³⁺, hydralazine rapidly forms a nitrogen-centered-DMPO adduct with a^N = 15.0G, a_β^H = 16.7G and a_β^N = 2.55G. While catalase has a very small inhibitory effect, superoxide dismutase completely inhibits the formation of the DMPO adduct. Mass spectral analysis of the adduct indicates that the hydralazyl radical is trapped with DMPO. Human red blood cells also catalyze the formation of a nitrogen-centered-DMPO adduct, a^N = 15.9G, a_β^H = 19.4G and a_β^N = 1.7G, which is different than that obtained with metal ions. DMPO-H adduct is also formed in the red cells from hydralazine.     

Studies on the phytoplankton ecology of the trondheemsfjord. II. Chloroplast pigments in relation to abundance and physiological state of the phytoplankton

The quantitative composition of the chloroplast pigments of phytoplankton sampled weekly at one station in the Trondheimsfjord was studied by circular paper chromatography throughout 18 months. The concentrations of total chlorophyll a (T-chl a obtained by the trichromatic method) as well as of chromatographically purified chlorophyll a (chl a) followed the variations in phytoplankton concentration. Two spring blooms and a weak autumn flowering of phytoplankton were clearly reflected in the pigment contents found, namely 14–16 mg T-chl a/m³ for the spring maxima, corresponding to nearly 300 mg T-chl a/m² for the euphotic zone; and 3–4 mg/m³ or 32 mg/m² for the autumn peak. The concentrations between blooms amounted to ≈ 1 mg T-chl a/m³, while concentrations down to 0.03 mg/m³ were found for winter samples.The content of T-chl a was high in diatom cells prior to a bloom (20–40 × 10^?9 mg/cell). During rapid growth (a more or less exponential phase) the cell content of chloroplast pigments decreased (to 5–10 × 10^?9 mg). No degradation product of chlorophylls could be detected during this phase and the percentage of chl a (of T-chl a) was high (70–80 %). At the peak of the bloom, and especially when the nitrate content in the surrounding water had been exhausted, low values for T-chl a were found ($\text{0.3–0.5 × 10}{}^{\mathrm{<mtext>9</mtext><mtext>?</mtext>}}\text{mg/cell}$). As soon as the cell counts started to fall, or even before the decline could be clearly detected, the percentage of chl a dropped (to 40-20 %) and derived chlorophylls (not phaeophytin a) were present in the samples. Model studies with cultured algae showed a similar behaviour.It is concluded that the proportion of chl a to T-chl a and the occurrence of chlorophyll derivatives in phytoplankton samples can give valuable information on the stage of development of the algal populations involved.     

Steady-state kinetics of electron transfer through cytochrome chain of uncoupled submitochondrial particles. II. Influence of pH on kinetics of electron transfer

pH Dependences of steady-state kinetic parameters of cytochrome chains of submitochondrial particles have been studied. It has been shown that the lifetimes of activated states (τ) of the pairs of cytochromes b → c₁ and a → a₃ have different pH dependences; those for the c₁ → c and c → a cytochrome pairs being similar. The rate constants for the non-activated state of the respiratory chains decreased for the b → c₁ pair and increased for the a → a₃ pair when the pH value was increased.The values of pK calculated from these dependences for the pairs b → c₁ and a → a₃ were 7.2 and 8.9, respectively. It has been supposed that the ratio of activated to non-activated electron carriers may be controlled by the local pH value in the mitochondrial membrane, the latter being dependent upon the rate of electron transfer. The kinetic model based on this assumption allows one to explain the experimental dependences on pH of the rate constants for cytochromes b → c, and a → a₃.The values of the diffusion rate constants for H⁺ and OH^? ions in the mitochondrial membrane estimated from these kinetic data obtained in this study weree 10⁴–10⁵ s^?1 and 10²–10³ s^?1, respectively.     

The mouse alpha-amylase multigene family. Sequence organization of members expressed in the pancreas, salivary gland and liver

The DNAs that specify the α-amylase messenger RNAs found in the pancreas, salivary gland and liver of mouse strain A have been isolated by molecular cloning in phage λ. Amylase clones were studied by mRNA/DNA hybrid analysis in the electron microscope, restriction endonuclease site mapping and DNA sequencing. The Amy-2^a gene, which specifies pancreatic α-amylase mRNA, measures 10·1 kb from cap to polyadenylation site and is interrupted by at least 9 intervening sequences. Amy-1^a, which specifies both salivary gland and liver α-amylase mRNAs contains at least 10 introns. The distance between the cap and polyadenylation sites used in the salivary gland and the liver measures 22·9 kb and 20 kb, respectively. Introns are located at very similar, if not identical, positions within comparable regions of Amy-1^a and Amy-2^a. The first intron of Amy-1^a, which interrupts sequences specifying 5′ non-translated regions of salivary gland and liver α-amylase mRNAs, has no counterpart in Amy-2^a. Some introns exhibit considerable sequence homology, suggesting that Amy-1^a and Amy-2^a have evolved by duplication from a common split ancestor sequence. Repetitive sequence elements occur in the introns and flanking regions of these genes. Gene titration by quantitative autoradiography reveals only one copy of Amy-1^a, but two copies of Amy-2^a per haploid mouse genome. In addition to Amy-1^a and Amy-2^a, several other amylase-like DNA sequences exist in the mouse genome. No gross rearrangements of amylase DNA sequences can be detected between germline DNA and that of various mouse tissues.     

Somatic variations on a yellow mutant in Nicotiana tabacum L. (a1+/a1a2+/a2) I. Non-reciprocal genetic events occurring in leaf cells

A greenish-yellow mutant was obtained after treatment of seeds of Nicotiana tabacum L. var. Xanthi n.c. with ethyl methanesulfonate (EMS). Two genetically independent mutations (a₁ and a₂) were isolated. The first mutation (a₁) antagonizes the function of its partially dominant a₁⁺ allele. The second mutation (a₂) is amorphous but strongly interacts with a₁.Among the nine possible genotypes at the two loci, five varied in somatic cells. The heterozygous state a₁⁺/a₁ strongly increased the frequency of both spontaneous and induced variations. However, two homozygotes also showed variations.Variants were isolated from induced and spontaneous non-reciprocal and reciprocal variations within paliside tissues by bud induction in vitro. They were genetically tested. In this first paper, only non-reciprocal variations are reported.Green variants from the greenish-yellow (J¹) dihybrid a₁⁺/a₁a₂⁺/a₂ clone had two genotypes: the first was due to true reversions of a₁ to a₁⁺, whereas the second was due to amorphous a₁⁰ mutations from a₁. These a₁⁰ mutations may well be deletions.The lightest yellow variants from J¹ were due to mutations either from a₁⁺ into a₁ or from a₂⁺ into a₂.Deletions at the a₁⁺?a₁ locus led to either yellow variations when a₁⁺ was lost, or to false reversions when the antagonistic allele a₁ was lost.Amorphous alleles at the a₁⁺?a₁ locus were also isolated from tissues other than J⁺. They gave zygotic lethality (s) that probably varied with the size of the deletions. Thus, true reversions and deletions at the a₁⁺?a₁ locus could be distinguished from one another by progeny tests.Other variants showed higher frequencies of spontaneous variations (instability). Somatic changes observed in these unstable systems were due to modifications at the marker loci. The genetic nature of this instability is not yet known.There is strong evidence that the genetic events involved in these non-reciprocal variations were deletions, conversions and point mutations. True reversions from a₁ into a₁⁺ and new mutations from a₁⁺ into a₁ were obtained only from a₁⁺/a₁. It was therefore supposed that the changes observed took place only in heterozygotes, and the conversion hypothesis was made. Attempts are being made to prove that conversions do exist in higher plants, and to find out if this process, as deletions, is induced by radiation.     

Distribution of hereditary blood groups among Indians in South America. II. In Peru

Blood specimens were procured from the following putatively pure Indians of the Peruvian rain forest: 90 Piro and 89 Campa on the Urubamba and Tambo rivers, 142 Shipibo and 14 Isconahua on the Rio Ucayali near Yarinacocha, 151 Aguaruna at Santa Maria de Nieva, where the Marañon and Nieva rivers join, and from 122 Ticuna and 9 Yagua near the Brazilian border on the Amazon. Specimens from highland Indians were obtained from 93 Aymará and 181 Quechua at Puno and environs. These 891 specimens were tested for antigens in the A-B-O, M-N-S-s, P, Rh-Hr, Lutheran, K-k, Lewis, Duffy, Kidd, and Diego (Di^a) systems, and for the Wright (Wr^a) aglutinogen. Serum samples from these bloods were tested for haptoglobins and transferrins and hemolysates were prepared and examined for hemoglobin types. Results for these tests with claculated gene frequencies are presented, for the most part, on appropriate tables. A map is included to show the locations of the populations from which blood samples were procured. As in South American Indians generally, frequencies are high for the O gene it being the only gene of the ABO system which appears in isolated jungle populations and the Aymará. Gene frequencies are usually high also for M, s, R¹ (CDe), R² (cDE), Lu^b, k, Le^H, and Fy^a; and low or absent for A, B, N, S, Mi^a, Vw, R^o (cDe), r (cde), Lu^a, K, Le¹, Fy^b, and Wr^a. The Diego (Di^a) gene is present but varies greatly in frequencies among tribes. Hp¹ gene frequencies vary from 0.44 to 0.69 among the Peruvian Indians tested. Transferrin CD was encountered in only one population i.e., in 3 of 86 Piro (gene frequency Tf^D= 0.02). All others were C. All Peruvian Indian bloods tested electrophoretically contained only hemoglobin (A) as a major component.     

Structure and expression of the Lyt-3 agene of C.AKR mice

The mouse Lyt-3 ^agene, which encodes the Lyt-3.1 T-cell surface alloantigen of the C.AKR strain, has been cloned, and the nucleotide sequence of its exons and more than 2 kb of 5 flanking sequence have been determined. The gene extends over approximately 16 kb of DNA and consists of six exons encoding leader, leader plus V-like domain, membrane-proximal, transmembrane, and cytoplasmic domains. The only difference between the coding region of the Lyt-3 ^agene and the cDNA sequences reported for Lyt-3 ^b(Nakauchi et al. 1987, Panaccio et al. 1987) is at position 77 of the mature protein where Lyt-3 ^aencodes serine and Lyt-3 ^bencodes arginine. This substitution must therefore be the basis for the serological distinction between the Lyt-3.1 and Lyt-3.2 alloantigens. Potential TATA and CAAT sequences, two Sp1 protein binding sites, two extended repeats of the dinucleotide, CA, a number of short inverted repeats, and an inverted segment of the mouse B1 repetitive sequence are found 5 to the Lyt-3 ^agene. Two consensus poly-A addition signals and a complete copy of the mouse B1 sequence are found 3 to the gene. Both B1-related regions are flanked by short direct repeats suggesting that they arose by an insertional mechanism. Cotransfection of the Lyt-3 ^agene together with a cloned Lyt-2 ^agene resulted in expression of both Lyt-2 and Lyt-3.1 on the surface of Ltk^– and BW5147 cells. Transfection of the Lyt-3 ^agene without Lyt-2 ^aled to expression of Lyt-3-related cellular RNA but did not result in surface expression of Lyt-3.1, suggesting that the Lyt-3 glycoprotein is not expressed on the cell surface in the absence of Lyt-2.     

Distribution of hereditary blood groups among Indians in South America. 3. In Bolivia

Blood samples were procured from the following populations of putatively pure Indians in Bolivia: 503 Aymará from the Altiplano and Yungas, 30 Chama, 11 Tacana, 14 Chácobo, 109 Itonama, 67 Moré, and 27 Sirionó from the Beni and lowland rainforest. Erythrocytes from these 761 specimens were tested for antigens in the A-B-O, M-N-S-s, P, Rh-Hr, Lutheran, Kell-Cellano, Lewis, Duffy, Kidd, and Diego systems, and for the Wright agglutinogen. The serum samples were tested for haptoglobins and transferrins; and hemolysates were prepared and examined for hemoglobin types. Results of these tests are presented as phenotypes and calculated gene frequencies on appropriate tables. A map is included to show the locations of the populations from which blood samples were obtained. Frequencies are generally high for the O gene, it being the only gene of the ABO system which appears in the Chama, Chácobo and Sirionó. The presence of A₁, A₂ or B genes in the Bolivian Indians is interpreted as being most probably of caucasoid introduction. Excepting the Sirionó the frequencies are high for M and low for N genes as is usual for Amerinds, the M gene being the only one detected in the Chama. The s gene frequency in high and the S low except in the small isolated Chácobo population in which S gene frequency is extremely high for Amerinds. Inbreeding and perhaps genetic drift in this small isolate may account for this aberrancy from normal. The Bolivian specimens presented the high frequencies for genes R¹ (CDe) and R² (cDE) and the low frequencies for genes r (cde) and R⁰ (cDe) usually observed in American Indians. The Lu^a factor was observed in only one of 120 Aymará at Santa Fe in the Yungas. The Lu^a factor, when observed in Amerinds, suggests foreign introduction of the responsible gene. Fy^a gene frequencies are consistently high and excepting the Aymará and Chama so also are Jk^a frequencies. Frequencies for the Diego (Di^a) factor vary from 3.70% in 27 Sirionó to 73.33% in 30 Chama. No K, Mi^a, Vw or Wr^a antigens were demonstrable in the Indian blood samples from Bolivia. Phenotypes and calculated gene frequencies for haptoglobins and transferrins are presented. All Bolivian Indian bloods tested electrophoretically contained only hemoglobin (A) as a major component.     

Periphyton productivity and radionuclide accumulation in the Columbia River,Washington, U.S.A.

Summary An investigation of the periphyton of the Columbia River below the Hanford Atomic Works, Washington, was conducted to study the relationships between productivity, radionuclide uptake, and environmental influences.Best correlations between the four biomass measurements were between dry weight, ash weight, and chlorophyll a. Net Production Rate varied from 0.005 to 0.070 mg dry weight/cm²/day and was closely related to chlorophyll a and also to solar energy.The accumulation of ³²P and ⁶⁵Zn was highly related to dry and ash weight and chlorophyll a. Low correlations were found between radionuclide accumulation and the radioisotope burden of the river. The data suggest that adsorption was the dominant mode of uptake.This paper is based on work performed under United States Atomic Energy Commission Contract AT(45-1)-1830.     

Radiation-induced human chromosome aberrations. II. Human in vitro irradiation compared to in vitro and in vivo irradiation of marmoset leukocytes

Whole blood cultures from humans and from the New World primate, Saguinus fuscicollis, were irradiated with various doses of 250 kV X-rays. The resulting centric ring plus dicentric aberration yields were fitted to the three models, Y = a+bD, Y = a+bD+cD², and Y = a+cD², by least squares regression. In both instances the best fit was to the model Y = a+bD+cD², with coefficients of the one- and two-track components for human and marmoset being: b = (0.78 ± 0.09)·10⁻³, c = (5.92 ± 0.31)·10⁻⁶, and b = (1.11 ± 0.36)·⁻³, c = (7.7 ± 1.7)·10⁻⁶, respectively.     

Organization of chlorophyll a in bilayer membranes. The chlorophyll a/dimyristoylphosphatidylcholine system

In order to investigate the relative importance of the hydrophobic and headgroup interactions of chlorophyll a in phospholipid bilayers, we have carried out differential scanning calorimetry (DSC) and deuterium (²H) and phosphorus (³¹P) nuclear magnetic resonance (NMR) experiments on the multilamellar system of chlorophyll a in dimyristoylphosphatidylcholine (DMPC). Compared to the phytol chain of chlorophyll and the previously reported distearoylphosphatidylcholine (DSPC), the acyl chains of DMPC are shorter in length by three and four carbons, respectively. A lowering in the phase-transition temperature was observed for the DMPC multilayers in the presence of chlorophyll a in the DSC thermograms and in the ³¹P chemical shift anisotropy measurements. These results, together with data on hydrophobic interactions as measured by ²H-NMR and on headgroup interactions as evidenced from ³¹P-NMR, suggest a phase diagram for the chlorophyll a/DMPC system in which phase separation readily occurs between a chlorophyll-rich compound phase and a chlorophyll-poor phospholipid phase. Compound formation appears to be important in the stabilization of chlorophyll a in bilayers with shorter chains.     

Bacteriophage T4 transfer RNA. I. Isolation and characterization of two-phage-coded nonsense suppressors

Two phage-coded nonsense suppressors, psuf_a⁺ and psu_b⁺, have been isolated and characterized. Both were isolated as pseudo-wild type revertants of phage strains which carry multiple amber mutations. psu_a⁺ is an amber suppressor which occurs at a frequency of 10⁻¹¹ to 10⁻¹² and is indistinguishable from wild type phage in its growth on both B and K strains of Escherichia coli bacteria. psu_b⁺ may be either an amber or an ochre suppressor, which occurs at a frequency of 10⁻⁷ to 10⁻¹⁰ and makes small plaques on B strains, but grows very poorly or not at all on K strains. Phage with the characteristics of psu_a⁺ occur in populations of psu_b⁺ phage at a frequency of 10⁻⁴. Both suppressors insert serine in response to the amber codon at an efficiency of about 45%.     

Variation,geographical distribution and genetical analysis of esterase isozymes in foxtail millet,Setaria italica (L.) P. Beauv.

Summary The esterase isozymes of 432 strains of foxtail millet, Setaria italica (L.) P. Beauv., collected from different areas throughout Eurasia, were investigated by gel isoelectric focusing. Five phenotypes were recognized, based on the combination of five major activity bands. Cross experiments among different phenotypes revealed these isozymes to be controlled by two codominant alleles and a null allele on the locus, Est-1, and three codominant alleles on another independent locus, Est-2. On locus Est-1, 388 strains had Est-1 ^a, 41 had Est-1 ^b and three had Est-1 ^null alleles. Est-1 ^a was widely distributed throughout Eurasia, while the distribution of Est-1 ^b and Est-1 ^null was distinctly restricted. On locus Est-2, 417 strains had Est-2 ^a, nine had Est-2 ^b and six had Est-2 ^c alleles. Est-2 ^a was widely distributed throughout Asia to Czechoslovakia, but was not detected in the western part of Europe. Est-2 ^b was found in all of the strains from the western part of Europe and in one of the Indian strains. Est-2 ^c was rarely found in Japan and China. The distribution of Est-2 ^a and -2 ^b might indicate some degree of phylogenetic differentiation between the Asian and the European strains. Polymorphism in both loci was observed only in Chinese strains.Contribution No. 30 from the Plant Germ-plasm Institute, Faculty of Agriculture, Kyoto University, Kyoto, Japan     

Effect of the desiccation tolerance and cryopreservation methods on the viability of Citrus limon L. Burm cv. Eureka seeds

Cryopreservation of the germplasm for long-term periods is of great importance to maintain the genetic resource. Argentina is one of the world's highest lemon producing country. The performance of different cooling/warming rates in the cryopreservation method of Citrus limon L. Burm cv. Eureka seeds and their influence on the interval of optimal moisture content in the desiccation stage were analyzed. Water sorption isotherm was determined and modeled using D'Arcy & Watt equation; it provided important information concerning the amounts of water associated to strong, weak and multimolecular binding sites along the sorption isotherm. Seeds tolerated a wide range of desiccation conditions (0.1<a_w<0.85) showing a high viability (>80%), however desiccation to 0.0526 g H₂O g⁻¹ d.b. (a_w = 0.0901) produced a significant loss of viability. Differential Scanning Calorimetry was used to identify the thermal transitions of lipids and water in the seed; enthalpies were used to calculate the unfrozen water fraction (0.19 g H₂O g⁻¹ d.b. corresponding to a_w = 0.64). Two cooling/warming rates were tested on desiccated seeds (0.11<a_w<0.85): i) 200 °C min⁻¹ (reached with seeds placed inside a closed cryogenic vial); ii) 1000 °C min⁻¹ (reached with aluminum-foiled seeds placed in a perforated cryogenic vial). For both methods, viability was maximum (83.3%) at a_w = 0.64. Lethal ice formation was responsible for the loss of viability at a_w>0.64 corresponding to the unfrozen water fraction. The use of higher cooling/warming rates enables a wider range of desiccation conditions (0.33<a_w<0.76) in cryopreservation procedures. This work contributes to the optimization of cryopreservation methods of economically important germplasm.     

1H nuclear magnetic resonance studies of histidine-containing di- and tripeptides. Estimation of the effects of charged groups on the pKa value of the imidazole ring.

The nmr titration curves of chemical shifts versus pH were observed for the protons of various histidine-containing di- and tripeptides. With these results, the macroscopic pK_a values and the chemical shifts intrinsic to each ionic species were determined by a computer curve-fitting based on a simple acid dissociation sequence. The pK_a value of the imidazole ring in N-acetyl-L -histidine methylamide was assumed to represent the intrinsic (or unperturbed) pK_a of the imidazole rings of histidine having peptide linkages at both the CO and NH sides. The pK_a values of the imidazole rings observed for most di- and tripeptides were reasonably reproduced by simple calculations using the intrinsic value and the perturbations due to the CO₂^? and NH₃⁺ groups located at various positions. Some other factors affecting the pK_a value of the imidazole ring are also discussed.     

Selective ingestion of phytoplankton by the bivalvesMytilus edulis L. andCerastoderma edule (L.)

Blue mussles (Mytilus edulis) and cockles (Cerastoderma edule) were collected in the Oosterschelde estuary (SW Netherlands) and exposed to diets consisting of the diatomPhaeodactylum tricornutum and silt. Algal concentrations were kept constant (20.10³ cells.ml^–1), and silt concentrations were varied between 3–80 mg.l^–1 (mussel experiments) and 20–120 mg.l^–1 (cockle experiments). Chlorophyll-a concentrations in the pseudofaeces were reduced compared to the diets, indicating selective ingestion of the algae. It was estimated that the mussels increase the ingestion of chlorophyll-a 2.0 times, and the cockles increase the chlorophyll-a ingestion 2.8 times, compared to the ingestion rate without selection. The selection coefficients were not affected by the SPM concentrations used. Phaeophytin-a concentrations in the faeces showed an increase as a consequence of the digestion of the ingested algae. Digestive efficiencies of chlorophyll-a varied between 36–92% The digestive efficiencies decreased with increasing SPM concentrations.Communication no. 541 of the Delta Institute for Hydrobiological Research.