Beef liver esterase. I. Isoelectric point and molecular weight

A sample of carboxylesterase has been purified from beef liver by the method of Runnegar et al. [Biochemistry8, 2013 (1969)]. The protein isoelectric point, pI was found to be 5.5, using the isoelectric focusing method. The molecular weight was found by equilibrium sedimentation to be 5.5 × 10⁴. Sedimentation velocity combined with diffusion gave a similar value. In high protein concentrations aggregation was not detected.     

Beef liver esterase. II. Kinetic properties

The kinetic parameters, k_cat and K_M, in beef liver esterase-catalyzed hydrolysis were determined for about 100 substrates, which can be classified in several groups: (1) In the ethyl ester series of fatty acids K_M decreases with elongation of the acid, while k_cat has a maximum value with pentanoate. (2) Alkyl acetates are better substrates as the alkyl moiety is longer, whereas esters with branched alkyl groups become worse substrates. (3) Aryl esters are very good substrates. (4) Esters of dicarboxylic acids are good substrates, but only one ester group is cleaved by the enzyme. Fumarate diester is susceptible to esterase hydrolysis, while maleate is not. (5) Esters of hydrophobic amino acids are very good substrates; the enzyme is not stereoselective and both the l and d stereoisomers are readily hydrolyzed. Branching at the β-carbon atom leads to loss of activity, and blocking of the amino group abolishes it. Fluoride ion and dl-malate esters are potent competitive inhibitors of the enzymic reaction. The optimal pH was found to lie between 8 and 8.5. The reaction rate increased between 5 and 40 °C then dropped sharply. The activity decreased at high salt concentration.     

THE FINE STRUCTURE OF THE NUCLEUS AND NUCLEAR ASSOCIATIONS OF DEVELOPING CARPOSPORANGIA IN POLYSIPHONIA NOVAE-ANGLIAE (RHODOPHYTA)1

A high degree of activity of the nuclei in the developing carposporangia of the red alga Polysiphonia novae-angliae Taylor is described. Profiles of the nucleus are greatly convoluted, resulting in a much increased surface area. Regions where endoplasmic reticulum substitutes for the nuclear envelope are frequently observed. Various cytoplasmic reserves are associated with the nucleus during carposporangium maturation. Lipid bodies, fibrillar bodies, and striated vesicles (or cylindrical bodies) may occur within the nucleoplasm beneath areas of the nuclear envelope substituted by ER. Granules of Floridean starch are observed in proximity to the outer surface of these same areas as well as the nuclear membrane. The homologous nature of the nuclear envelope with the ER is stressed. The role of the nucleus as being actively involved in the synthesis of materials is suggested.