The bacteriorhodopsin proton pump: effect of crosslinkings of lysine residues

All six available lysine residues in bacteriorhodopsin were amidinated with dimethyl-3,3'-dithiobispropionimidate, which is a crosslinking agent. The photocycle was studied by measuring light absorption and electric signals. The data show an essential change in the photocycle: instead of single components, the rise of the signal due to the M intermediate can be decomposed into two components, and the decay into three. The life-times and the intensities of these components and in general the proton pumping activity of bacteriorhodopsin depend only negligibly upon pH. Changes upon removing the crosslinks are not significantly different from those in the crosslinked samples. The lysine residues therefore may not be considered of primary importance in proton translocation.     

Observations on the nature of neurosecretion in the marine crab Portunus sanguinolentus (Herbst) (Crustacea: Brachyura)

Several types of NS cells were identified in Portunus sanguinolentus--five types (A, A', B, C and D) in the brain and thoracic ganglion, four types (A, B, C and D) in the commissural ganglia and four types (alpha, beta, gamma and delta) in the optic ganglia. The distribution of these NS cells is described. Cytochemically, the neurosecretory material in the NS cells has a carbohydrate moiety and is rich in disulphide groups, lipids, phospholipids and RNA. It contains a small amount of sulphydryl groups and protein-bound NH2 groups, but no tyrosine or tryptophan. The NS activity of the brain was found to be closely associated with the reproductive and moult cycles. Just before the initiation of vitellogenesis and moulting the NS cells display secretory hyperactivity. Axonal transport of NS material was also observed in the NS cells.     

Sex difference in susceptibility to teratogenic effect of maternal biotin deficiency in mouse embryos

When pregnant mice were fed biotin-deficient diets, cleft palate occurred more frequently in male fetuses than in female fetuses. Possible underlying mechanisms are speculated on, and some methodological problems in the analysis of sex-related differences in multiparous animals are discussed.     

Enzymes of nucleotide metabolism: the significance of subunit size and polymer size for biological function and regulatory properties

The 72 enzymes in nucleotide metabolism, from all sources, have a distribution of subunit sizes similar to those from other surveys: an average subunit Mr of 47,900, and a median size of 33,300. The same enzyme, from whatever source, usually has the same subunit size (there are exceptions); enzymes having a similar activity (e.g., kinases, deaminases) usually have a similar subunit size. Most simple enzymes in all EC classes (except class 6, ligases/synthetases) have subunit sizes of less than 30,000. Since structural domains defined in proteins tend to be in the Mr range of 5,000 to 30,000, it may be that most simple enzymes are formed as single domains. Multifunctional proteins and ligases have subunits generally much larger than Mr 40,000. Analyses of several well-characterized ligases suggest that they also have two or more distinct catalytic sites, and that ligases therefore are also multifunctional proteins, containing two or more domains. Cooperative kinetics and evidence for allosteric regulation are much more frequently associated with larger enzymes: such complex functions are associated with only 19% of enzymes having a subunit Mr less than or equal to 29,000, and with 86% of all enzymes having a subunit Mr greater than 50,000. In general, larger enzymes have more functions. Only 20% of these enzymes appear to be monomers; the rest are homopolymers and rarely are they heteropolymers. Evidence for the reversible dissociation of homopolymers has been found for 15% of the enzymes. Such changes in quaternary structure are usually mediated by appropriate physiological effectors, and this may serve as a mechanism for their regulation between active and less active forms. There is considerable structural organization of the various pathways: 19 enzymes are found in various multifunctional proteins, and 13 enzymes are found in different types of multienzyme complexes.     

Type C Niemann-Pick disease: spectrum of phenotypic variation in disruption of intracellular LDL-derived cholesterol processing

To investigate biochemical heterogeneity within Niemann-Pick type C disease (NPC), the two most characteristic abnormalities, namely (1) kinetics of LDL-stimulated cholesteryl ester formation and (2) intravesicular accumulation of LDL-derived unesterified cholesterol, evaluated by histochemical filipin staining, were studied in cultured skin fibroblasts from a population of 125 NPC patients. Profound alterations (esterification rates less than 10% of normal, very numerous and intensely fluorescent cholesterol-filipin granules) were demonstrated in 86% of the cases, depicting the 'classical' NPC phenotype. The remaining cell lines showed a graded less severe impairment and more transient delay in the induction of LDL-mediated cholesteryl esterification, along with an attenuated accumulation of unesterified cholesterol. In particular, cells from a small group (7%) of patients, which have been individualized as representative of a 'variant' phenotype, showed only slight alterations of esterification, restricted to the early phase of LDL uptake and undistinguishable from those in heterozygotes. In these cells, an abnormal cytochemical distribution of LDL-derived cholesterol, although moderate, was still evident provided rigorous experimental conditions were followed. A third, less clearly individualized group (7%), differing from the classical phenotype mostly by higher rates of cholesteryl ester formation, has been designated as an 'intermediary' phenotype to reflect a more difficult diagnosis of such patients. These findings have an important bearing with regard to diagnosis and genetic counselling, although the significance of such a phenotypic variation in terms of genetic heterogeneity has still to be demonstrated. A given biochemical phenotype was however a constant observation within a family (14 pairs of siblings tested so far). The unique feature of LDL-cholesterol processing alterations in NPC has been further established from comparative studies in Wolman disease and I-cell disease, showing normal or different intracellular distribution of unesterified LDL-derived cholesterol in the latter disorders. Correlation between biochemical and clinical NPC phenotypes was only partial, but a correlation between the severity of alterations in cholesterol processing and sphingomyelin catabolism could be established.