Diffusion of paramagnetically labeled proteins in cartilage: Enhancement of the 1-D NMR imaging technique

Quantifying the diffusive transport of large molecules in avascular cartila ge tissue is important both for planning potential pharmacological treatmen ts and for gaining insight into the molecular-scale structure of cartilage. In this work, the diffusion coefficients of gadolinium-DTPA and Gd-labeled versions of four proteins-lysozyme, trypsinogen, ovalbumin, and bovine ser um albumin (BSA) with molecular weights of 14,300, 24,000, 45,000, and 67,0 00, respectively-have been measured in healthy and degraded calf cartilage. The experimental technique relies on the effect of the paramagnetic on the relaxation properties of the surrounding water, combined with the time cou rse of a 1-dimensional spatial profile of the water signal in the cartilage sample. The enhanced technique presented here does not require a prior mea surement of the relaxivity of the paramagnetic compound in the sample of in terest. The data are expressed as the ratio of the diffusion coefficient of a compound in cartilage to its diffusion coefficient in water. For healthy cartilage, this ratio was 0.34 +/- 0.07 for Gd-DTPA, the smallest compound , and fell to 0.3 +/- 0.1 for Gd-lysozyme, 0.08 +/- 0.04 for Gd-trypsinogen , and 0.07 +/- 0.04 for Gd-ovalbumin. Gd-BSA did not appear to enter health y cartilage tissue beyond a surface layer. After the cartilage had been deg raded by 24-h trypsinization, these ratios were 0.60 +/- 0.03 for Gd-DTPA, 0.40 +/- 0.08 for Gd-lysozyme, 0.42 +/- 0.09 for Gd-trypsinogen, 0.16 +/- 0 .14 for Gd-ovalbumin, and 0.11 +/- 0.05 for Gd-BSA. Thus, degradation of th e cartilage led to increases in the diffusion coefficient of up to fivefold for the Gd-labeled proteins. These basic transport parameters yield insigh ts on the nature of pore sizes and chemical-matrix interactions in the cart ilage tissue and may prove diagnostically useful for identifying the degree and nature of damage to cartilage. (C) 2001 Academic Press.