Specific and non-specific folate binding protein in normal and malignant human tissues

Abstract

Binding of tritiated folic acid by supernatants prepared from extracts of normal and leukaemic leucocytes, normal mucosa, and malignant tumours from different parts of the gastrointestinal tract has been measured using Sephadex-gel filtration and albumin-coated charcoal techniques. Non-specific binding (measured by Sephadex G-75 gel filtration) was almost invariably greater than specific binding measured by albumin-coated charcoal separation of bound and unbound folate. In nine normal leucocyte extracts, binding measured by Sephadex G-75 filtration ranged from 1.3 to 18.2 (mean 8.2) pg/mg protein and by albumin-coated charcoal from 1.0 to 14.8 (mean 6.7) pg/mg protein. Raised specific binding was found in the extracts from leucocytes of eight of 14 patients with chronic granulocytic leukaemia, in four substantially so (389, 121, 108, 59.7 pg/mg protein), but was only marginally increased in one of eight cases of acute myeloid leukaemia and in two of five cases of chronic lymphocytic leukaemia. Binding was normal in the extracts of all three cases of acute lymphoblastic leukaemia tested. Among the tissues of the gastrointestinal tract binding was greatest by the duodenal mucosa and liver. Extracts of carcinoma of the stomach and colon bound greater amounts of (3)H-folic acid than the corresponding normal mucosal extracts but the differences were not large. Sephadex G-200 gel chromatography showed more than one binding peak in the extracts of liver and duodenum but only one peak in the other tissues of the gastrointestinal tract, and only one peak, of molecular weight either about 50 000 or over 200 000, in the leucocyte extracts.

Evidence for a Folic Acid Binding Protein in Human Cell Membrane

“…These results show that human LPM are able to bind labelled folic acid, confirming previous data [6,22], Since folate binding proteins are also present inside liver cells [7] purity of the membrane preparation was essential for this study. The absence of G-6-PD and SD from the LPM and other membrane preparations and the absence of other cellular organelles, including leuko cyte granules on light microscopy, excluded the possibility that binding detected here was due to cytoplasmic contamination.…”

Thermodynamics and Solvent Effects on Substrate and Cofactor Binding in Escherichia coli Chromosomal Dihydrofolate Reductase

“…A corollary of this model is that if a weak interaction occurs between the osmolytes and DHF and/or folate, then perhaps folate and/or DHF may bind to various proteins and/or cellular components. Some support for this corollary comes from various examples of nonspecific folate binding to proteins − as well as specific binding of folate to the central cavity in hemoglobin . Also, the Record group has experimentally found that betaine can compete with water and interact with amide and cationic nitrogens as well as aromatic hydrocarbons in small molecules .…”

Weak Interactions between Folate and Osmolytes in Solution

“…In an extension of our model, we ask if it is possible that folate may bind to various macromolecules. Other examples of weak binding of folate to various proteins have been noted. − Also, folate has been crystallized in the central cavity of hemoglobin . Most recently, a mass spectroscopy “allosterome” approach has searched for metabolite–protein interactions and found that folate inhibits glutamate dehydrogenase, glucose-6-phosphate dehydrogenase, and NAD kinase .…”