An invertase inactivator in maize endosperm and factors affecting inactivation

A protein present in the developing endosperm of maize (Zea mays L.) causes a loss of invertase activity under certain conditions of incubation. This protein, designated an inactivator, inactivates invertase I of maize even in the presence of other proteins. No inactivation of invertase II of maize or yeast invertase has been observed. The inactivator and invertase I are found only in the endosperm. The quantity of inactivator increases in the normal endosperm during development while invertase I activity decreases. However, the altered levels of invertase I activity in several endosperm mutant lines do not result from different quantities of inactivator. The inactivator can decrease invertase I activity during a preincubation period before addition of sucrose; inactivation is noncompetitive. Invertase I activity decreases curvilinearly with an increase in inactivator concentration. At high buffer concentrations or low inactivator concentrations in the reaction mixture, a latent period is observed when invertase I is not inactivated. Inactivation increases with an increase in temperature and a decrease in pH.     

Osmotic alterations of the lysosomal system during rat liver perfusion: Reversible suppression by insulin and amino acids

Livers from nonfasted rats were perfused in situ under conditions known from previous studies in this laboratory to increase or decrease overall endogenous proteolysis. At the termination of the experiments, lysosomal alterations were evaluated by the increase in free acid phosphatase or N-acetyl-β-D-glucosaminidase that occurred when tissue homogenates were subjected to osmotic shock in hypotonic sucrose. In control perfusions, osmotic sensitivity increased spontaneously over unperfused values, reaching maximum by 60 min or earlier. Additions of insulin, amino acid mixtures, or cycloheximide in amounts known to suppress proteolysis prevented this spontaneous perfusion effect or, when added at 60 min, rapidly reversed it. Glucagon alone during perfusion did not increase osmotic sensitivity further; however, stimulation with glucagon was observed when the perfusion effect was suppressed by insulin or cycloheximide. Anoxia, induced by gassing with nitrogen instead of oxygen, markedly reduced the perfusion effect and also doubled the amount of free acid phosphatase in the initial isotonic homogenates. Total acid phosphatase activities in the perfusion experiments were not significantly different from unperfused values and, with the exception of the anoxia perfusions, the amounts of free enzyme present in the initial isotonic sucrose homogenates did not change.     

Occurrence of soluble glycosyltransferases in human amniotic fluid

Human amniotic fluid obtained by amniocentesis during the third trimester of pregnancy was found to contain glycosyltransferases for the transfer of galactose, N-acetylgalactosamine, N-acetylglucosamine and sialic acid from their nucleotide derivatives to various exogenous protein and small molecular weight acceptors. The specific activity of the galactosyl- and N-acetylgalactosaminyl transferases was found to be 30 to 40 times higher in amniotic fluid as compared to serum. The specific activity of N-acetylglucosaminyl- and sialyl transferases was only 3 to 6 fold higher in amniotic fluid.     

Two Additional Phosphorylases in Developing Maize Seeds

Two additional phosphorylases (III and IV) have been detected in developing seeds of maize. Phosphorylase IV is found only in the embryo (with scutellum). It is also present in the embryo of the germinating seed where its activity is 90-fold greater than the activity in the developing embryo 22 days after pollination. Phosphorylase IV is eluted from a DEAE-cellulose column in the same fraction as phosphorylase I of the endosperm, and the 2 enzymes are similar in many respects. Phosphorylase IV is distinguished from phosphorylase I by electrophoretic mobility, by pH optimum, and because its properties are not affected by the shrunken-4 mutation.Phosphorylase III is found both in the endosperms and embryos of developing seeds. Activity for this enzyme is not detected in crude homogenates nor eluates from a DEAE-cellulose column apparently because it complexes with a non-dialyzable, heat-labile inhibitor. High activity is found after protamine sulfate fractionation. Phosphorylase III is bound to protamine sulfate and is then removed by washing with 0.3 m phosphate buffer. Phosphorylase III activity in the endosperm is not detectable 8 days after pollination but is present 12 days after pollination. Phosphorylase III differs from phosphorylases I, II, and IV in several respects-pH optimum, pH-independent ATP inhibition, time of appearance in the endosperm, and because purine and pyrimidine nucleotides are equally inhibitory. In common with phosphorylase II, phosphorylase III apparently does not require a primer to initiate the synthesis of an amylose-like polymer.