Objective: The accumulation of old and damaged proteins likely contributes to complications of diabetes, but currently no methodology is available to measure the relative age of a specific protein alongside assessment of posttranslational modifications (PTM). To accomplish our goal of studying the impact of insulin deficiency and hyperglycemia in type 1 diabetes upon accumulation of old damaged isoforms of plasma apolipoprotein A-1 (ApoA-1), we sought to develop a novel methodology, which is reported here and can also be applied to other specific proteins.
Research design and methods: To label newly synthesized proteins, [ring-(13)C(6)]phenylalanine was intravenously infused for 8 h in type 1 diabetic participants (n = 7) during both insulin treatment and 8 h of insulin deprivation and in nondiabetic participants (n = 7). ApoA-1 isoforms were purified by two-dimensional gel electrophoresis (2DGE) and assessment of protein identity, PTM, and [ring-(13)C(6)]phenylalanine isotopic enrichment (IE) was performed by tandem mass spectrometry.
Results: Five isoforms of plasma ApoA-1 were identified by 2DGE including ApoA-1 precursor (pro-ApoA-1) that contained the relatively highest IE, whereas the older forms contained higher degrees of damage (carbonylation, deamidation) and far less IE. In type 1 diabetes, the relative ratio of IE of [ring-(13)C(6)]phenylalanine in an older isoform versus pro-ApoA-1 was higher during insulin deprivation, indicating that de novo synthesized pro-ApoA-1 more rapidly accumulated damage, converting to mature ApoA-1.
Conclusions: We developed a mass spectrometry-based methodology to identify the relative age of protein isoforms. The results demonstrated accelerated oxidative damage to plasma ApoA-1, thus offering a potential mechanism underlying the impact of poor glycemic control in type 1 diabetic patients that affects a patient's risk for vascular disease.