What is "dithreitol"?
For the "shotgun" approach, saliva samples ($\pu{1 mL}$) were pre-fractionated with the use of Millipore ultracentrifuge filters (Millipore Corp., Billerica, MA, USA). Individual fractions were treated with $\pu{10 mM}$ dithreitol for $\pu{30 min}$, and then $\pu{50 mM}$ iodoacetamide for $\pu{30 min}$.


This reaction sequence in Ref.1 has a clear intention: First, break disulfide bonds in saliva proteins using a thiol exchanging reagent and then trap the recently formed thiol groups by iodoacetamide (Ref.2). According to Ref.2:


  Alkylation of cysteine thiols with iodo, bromo or chloro substituted acetic acid or acetamide is a classic approach that has been exploited since the 1930′s.


Thus, it is safe to assume that this second reaction is intended to trap thiol group as quickly as possible (The relative reaction rates between glutathione and these halogenated acetates are $100:60:1$ for iodo-, bromo- and chloro-acetates respectively; Ref.3). 

Also keep in mind that disulfide reduction to corresponding thiol is accomplished primarily by thiol exchange type reagents such as dithiothreitol (DTT) or 2-mercaptoethanol (ME). Thus, it is also safe to assume that dithreitol mentioned in Ref.1 is definitely a thiol exchanging reagent, possibly DTT. Therefore, I think it is a chain mistake as M. Farooq suggested in his comment. 

References:


S. Hu, Y. Li, J. Wang, Y. Xie, K. Tjon, L. Wolinsky, R. R. O. Loo, J. A. Loo, D. T. Wong, "Human Saliva Proteome and Transcriptome," J. Dent. Res. 2006, 85(12), 1129–1133 (DOI: 10.1177/154405910608501212).
Jakob R. Winther, Colin Thorpe, "Quantification of Thiols and Disulfides," Biochimica et Biophysica Acta 2014, 1840(2), 838-846 (https://doi.org/10.1016/j.bbagen.2013.03.031).
Frank Dickens, "CLII. Interaction of halogenacetates and SH compounds. The reaction of halogenacetic acids with glutathione and cysteine. The mechanism of iodoacetate poisoning of glyoxalase," Biochem J. 1933, 27(4), 1141–1151 (doi: 10.1042/bj0271141).