Apolipoprotein C-1 inhibits the hydrolysis by phospholipase A2 of phospholipids in liposomes and cell membranes.

A small polypeptide isolated from human serum inhibits the action of phospholipase A2 on dipalmitoylglycerol phosphocholine vesicles. Sequence analysis revealed the protein to be apolipoprotein C-1, a major component of very light-density lipoprotein. The inhibiting efficiency is increased by one order of magnitude after 10 min preincubation of the protein with the substrate, but not the enzyme.

Increase in pancreatic lipase and trypsin activity and their mRNA levels in streptozotocin-induced diabetic rats.

The pancreatic lipase and trypsin activities in streptozotocin-induced diabetic rats were determined as well as the relative levels of mRNA coding for these proteins. It was found that after development of diabetes, the activities of pancreatic lipase and trypsin were significantly increased by 105% and 52%, respectively, accompanied by an increase in the levels of lipase and trypsinogen mRNA by 98% and 49%, respectively, while amylase activity and its mRNA were significantly decreased. The alteration of lipase, trypsin, and amylase activities and their mRNA in diabetic rats were all normalized by replacement of insulin.

Are phospholipid-binding proteins in vivo phospholipase inhibitors?

Apo C1, a 6.5 kD protein component of VLDL, inhibits the hydrolysis of DPPC vesicles by pancreatic PLA2 and the hydrolysis of cellular phospholipids by endogenous phospholipases by interaction with the substrate. It is suggested that apo C1 and similar phospholipid-binding proteins modulate in vivo phospholipase activity, because they do so in cell homogenates with concentrations approximately equal to those found in the plasma.

Translational control of anionic trypsinogen and amylase synthesis in rat pancreas in response to caerulein stimulation.

Infusion of rats with optimal doses of caerulein for up to 24 hr resulted in divergent changes in protein synthesis in the exocrine pancreas: a 3-fold increase in synthesis of anionic trypsinogen and a 75% decrease in synthesis of amylase. Lipase synthesis showed no change. Rates of total protein synthesis increased 2-fold, while DNA, RNA, and poly(A)+ mRNA concentrations were unchanged during hormonal stimulation.

Transfer of cytochrome b 5 and NADPH cytochrome c reductase between membranes.

NADPH-cytochrome c reductase also reduces cytochrome b 5. The reduction is very slow when the proteins are in solution or bound to different membranes. Only when both proteins share a common membrane, is cytochrome b 5 reduced rapidly by NADPH.

The reactivities of tyrosine and tryptophan residues in lipid-bound cytochrome b5.

Purified cytochrome b5 from rabbit liver microsomes was bound to liposomes prepared from microsomal lipids. Tyrosyl and tryptophyl side chains of the protein were modified by water-soluble reagents and the reactivities of these amino acid residues in the liposome-bound cytochrome b5 were compared to those of the free protein. At pH 13, 80% of the tyrosines in lipid-free cytochrome b5 ionized immediately, whereas in the lipid-bound protein only 65% ionized within the first minute.

Action of 1-fluoro-2,4-dinitrobenzene on passive ion permeability of the human red blood cell.

Dinitrofluorobenzene (DNFB) inhibits the penetration of anions such as sulfate, phosphate, succinate, and lactate, and facilitates the penetration of cations such as K(+) and Na(+). The phlorizin-glucose insensitive fraction of erythritol permeability is not affected by the agent. The effects of DNFB on ion permeability are similar to those of more specific amino reactive agents like trinitrobenzene sulfonate and 2-methoxy-5-nitrotropone.