Isoelectric point region pI≈7.4 as a treasure island of abnormal proteoforms in blood

Abstract

Theoretical distribution of isoelectric points (pI values) of human blood proteins exhibits multimodality with a deep minimum in the range between pI 7.30 and 7.50. Considering that the pH of human blood is 7.4±0.1, normal forms of human proteins tend to eschew this specific pI region, thus avoiding charge neutrality that can result in enhanced precipitation. However, abnormal protein isoforms (proteoforms), which are the hallmarks and potential biomarkers of certain diseases, are likely to be found everywhere in the pI distribution, including this "forbidden" region. Therefore, we hypothesized that damaging proteoforms characteristic for neurodegenerative diseases are best detected around pI≈7.4. Blood serum samples from 14 Alzheimer's disease patients were isolated by capillary isoelectric focusing and analyzed by liquid chromatography hyphenated with tandem mass spectrometry. Consistent with the pI≈7.4 hypothesis, the 8 patients with fast memory decline had a significantly (p<0.003) higher concentration of proteoforms in the pI=7.4±0.1 region than the 6 patients with a slow memory decline. Moreover, protein compositions differed more from each other than for any other investigated pI region, providing absolute separation of the fast and slow decliner samples. The discovery of the "treasure island" of abnormal proteoforms in form of the pI≈7.4 region promises to boost biomarker development for a range of diseases.

ExoCAS-2: Rapid and Pure Isolation of Exosomes by Anionic Exchange Using Magnetic Beads

“…Adjusting the pH of an elution buffer below pI induces positive charge of protein and vice versa. However, various plasma proteins have a wide range of isoelectric points (pI) from to 5–9 [ 29 , 30 ].…”

The Biologics Revolution and Endotoxin Test Concerns

“…Sometimes it is good to have generalizations: "Theory predicts that the solubility of a protein will be minimal near the pI" (Shaw et al) [166] Many proteins will precipitate out when pH equals their pI. For this reason the pH of natural blood proteins avoids the 7.4 spot (as do all of the mAbs shown above, as the closest are just above 7.5) and, interestingly, some have theorized that this pI/pH phenomenon plays a role in some diseases in which malformed proteins accumulate: "abnormal proteoforms in the blood", including those "such as Alzheimer's disease (AD), Amyotrophic lateral sclerosis, prion disease, Creutzfeldt-acob disease, Parkinson's disease (PD), amyloidosis and a wide range of other disorders" [167].…”