Francis A. Jacobs

Detection of low molecular weight copper(II) and zinc(II) binding ligands in ultrafiltered milks-the citrate connection.

Low molecular weight zinc(II) and copper(II) binding ligands were detected in ultrafiltered human, bovine, and goat milk by the application of the method of modified gel chromatography. Human milk contains at least three detectable low molecular weight copper binders, whereas bovine and goat milk contain at least two. All three milks show two copper binding peaks with the same elution volumes. Zinc chromatograms were less specific than copper. Zinc showed only a single detectable low molecular weight binding ligand common to all three milks. Elution volumes of both zinc (II) and copper(II) citrate and picolinate systems were measured. Elution volumes of both copper II and zinc(II) citrate complexes are identical to elution volumes of an intense peak observed with all three milks; it is reasonable to assume that at least part of this peak corresponds to citrate. Human milk alone has a relatively intense binding peak for copper(II) at the same elution volume as the glutamate complex. Human and goat milk have another low intensity copper(II) binding ligand peak at the same elution volume; a number of amino acid complexes have binding peaks at this position. No peak characteristic of the zinc(II) or copper(II) picolinate systems could be found with any of the milks.

Solubility of calcium and zinc in model solutions based on bovine and human milks

The solubility of calcium, magnesium, and zinc in model solutions based on the low molecular weight components of bovine and human milks was examined over a pH range similar to that found in the human digestive system. Zinc was removed from solution in all models as calcium phosphates precipitated. The pH at which precipitable calcium phosphates formed was altered by the concentration of inorganic phosphate. All calcium and zinc in a model based on human milk remain in solution up to pH 6.5 while in a model based on bovine milk they were in solution up to pH 5. The use of simple model solutions may provide information useful for understanding the different bioavailabilities of minerals from skimmed bovine and human milks.

DYNAMICS OF AMINO ACID TRANSPORT IN THE INTACT INTESTINE.

Summary Net active absorption of amino acids, as mixtures at blood level, has been demonstrated. Both absorption and efflux occur simultaneously across the intact intestine. The transport of amino acids is an active process and is indeed dynamic.

Effect of ph on the speciation and solubility of divalent metals in human and bovine milks

Computer models estimated the ligand speciation and solubility of calcium, magnesium, zinc, and copper over a pH range for low molecular weight fractions characteristic of either human or bovine milks. Above pH 4 calcium is the only metal predicted to precipitate. Most of the remaining soluble calcium, magnesium, and zinc should be complexed with citrate. The solubility of calcium, magnesium, and zinc in human and bovine milks was measured experimentally from pH 2 to 7. The solubility of all three metals decreased as the pH increased. Calcium and zinc were soluble over a narrower pH range in bovine milk than in human milk. Increasing the levels of either calcium or inorganic phosphate alone in decaseinated human milk did not affect the solubility of zinc, but when both calcium and inorganic phosphate were added at levels comparable to bovine milk the solubility of zinc decreased at the higher pHs. The decreased solubility of zinc in skimmed milks in pHs characteristic of the small intestine is likely due to coprecipitation of zinc with calcium phosphate--a reaction not predicted for milk systems from known chemical solubility product data.

Effects of phlorizin on glucose transport into blood and lymph

Abstract Phlorizin was administered by oral and intraperitoneal routes to rats prepared with cannulated mesenteric lymph ducts. Oral phlorizin (fed with glucose by stomach-tube) depressed the uptake of fed glucose into both blood and lymph. No inhibition of water transport occurred while glucose transport was decreased, yet very high levels of oral phlorizim (220 μmoles) would block water transport to lymph from the intestine. Intraperitoneally injected phlorizin did not show an effect on glucose transported by lymph or blood, but did promote glycosuria. Typical findings are presented in the text.

Low Molecular Weight Copper-Binding Ligands in Human Bile

Abstract The aim of this study was to detect and identify major low molecular weight (< 10,000) copper-binding ligands in human bile. Modified gel chromatography was used as the method of ligand detection because it ensures the detection of labile as well as inert metal-ligand complexes. Conjugated bilirubin, peptides, and amino acids, primarily glycine, were isolated as the major ligands. In contrast to the other copper-binding ligands, the peptides were poor zinc binders, suggesting the possibility that they may confer necessary specificity to trace metal elimination.

Transport of amino acids via the mesenteric lymph duct in rats.

Summary It was demonstrated that dietary amino acids appear in the mesenteric lymph and are transported from the intestinal lumen both as free amino acids and newly synthesized protein via the lymphatic system. The amino acid profiles in intestinal lymph are compared to those in plasma and the intestinal mucosa. There seems to be a depression of lymph amino acid levels during the early phase of intestinal absorption which returns to normal in the later phase. This depression occurs at the same time the newly synthesized protein appears in the lymph.

Continuous radioactivity monitoring of perfusion in the small intestine of the intact animal.

The phenomenon of intestinal absorption and transport has been studied by a variety of techniques. Since in vitro studies are removed from the physiological state and since it is very difficult, if not impossible, to simulate the steady state in the living intact animal, various modifications of in vivo systems have been developed. We have reported [1] an in vivo perfusion system in which continuous absorption studies can be carried out using a single anesthetized intact small animal (rat), permitting the determination of continuous absorption patterns from the intestinal lumen in the same living animal. Sampling is possible at any elected time interval. The system described is a recirculating one. We have described a modification of the system, a single-pass perfusion [2’ 3] of the lumen with no recirculation of the solution perfused. Both of the systems have certain limitations in their operation. In the recirculating system samples drawn from the mixed perfusate during the experimental period bring about a diminution of the total volume perfused. In the single-pass system samples can be collected over any selected time interval; however, the quantities of material which are required for analyses and the slight gradient in concentration can pose technical problems.

Intraluminal chemistry of zinc in milks

The solubility of both free and low molecular weight ligand complexed calcium, magnesium, and zinc in skimmed human and bovine milks over intestinal luminal pH ranges (∼3–7) was measured using ultrafiltration techniques. Some of the experimental difficulties associated with the separation of labile metal ion ligand components from milks by ultrafiltration techniques are discussed. Experimental methods designed to minimize interferences in mineral ultrafiltrations from milks are outlined. Mineral solubilities in skimmed human and bovine milks are compared to data obtained in a previous study using milk models. The solubility of zinc in both skimmed bovine and bovine model milks is less than in human and human model milks at the higher pHs, characteristic of the luminal region where zinc absorption is thought to occur. The decrease in zinc solubility is caused by the coprecipitation of zinc with calcium phosphate, particularly in bovine milk samples. If solubility at the higher pHs is a requisite for zinc absorption then the enhanced bioavailability of zinc from human milk may be related to the detrimental element-compound interaction discussed in this study.

Coprecipitation modulates the solubility of minerals in bovine milk

The solubilities of zinc, iron, copper, magnesium, calcium, inorganic phosphate, and citrate in milk decreased when acidic milk preparations were neutralized. In decaseinated bovine milk soluble zinc, iron, and copper were reduced 90%, 60%, and 50%, respectively, as the pH was raised from 4 to 7. Simultaneous precipitation of minerals and citrate was confirmed by analysis of washed precipitate. We propose that the diminished solubilities of zinc, iron, copper, magnesium, and citrate are linked to the precipitation of calcium phosphate through one or more mechanisms of coprecipitation. Such control on mineral solubility may have an impact upon mineral absorption from milk.